LU-1070 agl: update lock state when AGL upcall

[fs/lustre-release.git] / lustre / include / cl_object.h

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/*
 * Copyright (c) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
 * Use is subject to license terms.
 *
 * Copyright (c) 2011, 2012, Whamcloud, Inc.
 */
/*
 * This file is part of Lustre, http://www.lustre.org/
 * Lustre is a trademark of Sun Microsystems, Inc.
 */
#ifndef _LUSTRE_CL_OBJECT_H
#define _LUSTRE_CL_OBJECT_H

/** \defgroup clio clio
 *
 * Client objects implement io operations and cache pages.
 *
 * Examples: lov and osc are implementations of cl interface.
 *
 * Big Theory Statement.
 *
 * Layered objects.
 *
 * Client implementation is based on the following data-types:
 *
 *   - cl_object
 *
 *   - cl_page
 *
 *   - cl_lock     represents an extent lock on an object.
 *
 *   - cl_io       represents high-level i/o activity such as whole read/write
 *                 system call, or write-out of pages from under the lock being
 *                 canceled. cl_io has sub-ios that can be stopped and resumed
 *                 independently, thus achieving high degree of transfer
 *                 parallelism. Single cl_io can be advanced forward by
 *                 the multiple threads (although in the most usual case of
 *                 read/write system call it is associated with the single user
 *                 thread, that issued the system call).
 *
 *   - cl_req      represents a collection of pages for a transfer. cl_req is
 *                 constructed by req-forming engine that tries to saturate
 *                 transport with large and continuous transfers.
 *
 * Terminology
 *
 *     - to avoid confusion high-level I/O operation like read or write system
 *     call is referred to as "an io", whereas low-level I/O operation, like
 *     RPC, is referred to as "a transfer"
 *
 *     - "generic code" means generic (not file system specific) code in the
 *     hosting environment. "cl-code" means code (mostly in cl_*.c files) that
 *     is not layer specific.
 *
 * Locking.
 *
 *  - i_mutex
 *      - PG_locked
 *          - cl_object_header::coh_page_guard
 *          - cl_object_header::coh_lock_guard
 *          - lu_site::ls_guard
 *
 * See the top comment in cl_object.c for the description of overall locking and
 * reference-counting design.
 *
 * See comments below for the description of i/o, page, and dlm-locking
 * design.
 *
 * @{
 */

/*
 * super-class definitions.
 */
#include <lu_object.h>
#include <lvfs.h>
#ifdef __KERNEL__
#        include <linux/mutex.h>
#        include <linux/radix-tree.h>
#endif

struct inode;

struct cl_device;
struct cl_device_operations;

struct cl_object;
struct cl_object_page_operations;
struct cl_object_lock_operations;

struct cl_page;
struct cl_page_slice;
struct cl_lock;
struct cl_lock_slice;

struct cl_lock_operations;
struct cl_page_operations;

struct cl_io;
struct cl_io_slice;

struct cl_req;
struct cl_req_slice;

/**
 * Operations for each data device in the client stack.
 *
 * \see vvp_cl_ops, lov_cl_ops, lovsub_cl_ops, osc_cl_ops
 */
struct cl_device_operations {
        /**
         * Initialize cl_req. This method is called top-to-bottom on all
         * devices in the stack to get them a chance to allocate layer-private
         * data, and to attach them to the cl_req by calling
         * cl_req_slice_add().
         *
         * \see osc_req_init(), lov_req_init(), lovsub_req_init()
         * \see ccc_req_init()
         */
        int (*cdo_req_init)(const struct lu_env *env, struct cl_device *dev,
                            struct cl_req *req);
};

/**
 * Device in the client stack.
 *
 * \see ccc_device, lov_device, lovsub_device, osc_device
 */
struct cl_device {
        /** Super-class. */
        struct lu_device                   cd_lu_dev;
        /** Per-layer operation vector. */
        const struct cl_device_operations *cd_ops;
};

/** \addtogroup cl_object cl_object
 * @{ */
/**
 * "Data attributes" of cl_object. Data attributes can be updated
 * independently for a sub-object, and top-object's attributes are calculated
 * from sub-objects' ones.
 */
struct cl_attr {
        /** Object size, in bytes */
        loff_t cat_size;
        /**
         * Known minimal size, in bytes.
         *
         * This is only valid when at least one DLM lock is held.
         */
        loff_t cat_kms;
        /** Modification time. Measured in seconds since epoch. */
        time_t cat_mtime;
        /** Access time. Measured in seconds since epoch. */
        time_t cat_atime;
        /** Change time. Measured in seconds since epoch. */
        time_t cat_ctime;
        /**
         * Blocks allocated to this cl_object on the server file system.
         *
         * \todo XXX An interface for block size is needed.
         */
        __u64  cat_blocks;
        /**
         * User identifier for quota purposes.
         */
        uid_t  cat_uid;
        /**
         * Group identifier for quota purposes.
         */
        gid_t  cat_gid;
};

/**
 * Fields in cl_attr that are being set.
 */
enum cl_attr_valid {
        CAT_SIZE   = 1 << 0,
        CAT_KMS    = 1 << 1,
        CAT_MTIME  = 1 << 3,
        CAT_ATIME  = 1 << 4,
        CAT_CTIME  = 1 << 5,
        CAT_BLOCKS = 1 << 6,
        CAT_UID    = 1 << 7,
        CAT_GID    = 1 << 8
};

/**
 * Sub-class of lu_object with methods common for objects on the client
 * stacks.
 *
 * cl_object: represents a regular file system object, both a file and a
 *    stripe. cl_object is based on lu_object: it is identified by a fid,
 *    layered, cached, hashed, and lrued. Important distinction with the server
 *    side, where md_object and dt_object are used, is that cl_object "fans out"
 *    at the lov/sns level: depending on the file layout, single file is
 *    represented as a set of "sub-objects" (stripes). At the implementation
 *    level, struct lov_object contains an array of cl_objects. Each sub-object
 *    is a full-fledged cl_object, having its fid, living in the lru and hash
 *    table.
 *
 *    This leads to the next important difference with the server side: on the
 *    client, it's quite usual to have objects with the different sequence of
 *    layers. For example, typical top-object is composed of the following
 *    layers:
 *
 *        - vvp
 *        - lov
 *
 *    whereas its sub-objects are composed of
 *
 *        - lovsub
 *        - osc
 *
 *    layers. Here "lovsub" is a mostly dummy layer, whose purpose is to keep
 *    track of the object-subobject relationship.
 *
 *    Sub-objects are not cached independently: when top-object is about to
 *    be discarded from the memory, all its sub-objects are torn-down and
 *    destroyed too.
 *
 * \see ccc_object, lov_object, lovsub_object, osc_object
 */
struct cl_object {
        /** super class */
        struct lu_object                   co_lu;
        /** per-object-layer operations */
        const struct cl_object_operations *co_ops;
};

/**
 * Description of the client object configuration. This is used for the
 * creation of a new client object that is identified by a more state than
 * fid.
 */
struct cl_object_conf {
        /** Super-class. */
        struct lu_object_conf     coc_lu;
        union {
                /**
                 * Object layout. This is consumed by lov.
                 */
                struct lustre_md *coc_md;
                /**
                 * Description of particular stripe location in the
                 * cluster. This is consumed by osc.
                 */
                struct lov_oinfo *coc_oinfo;
        } u;
        /**
         * VFS inode. This is consumed by vvp.
         */
        struct inode             *coc_inode;
};

/**
 * Operations implemented for each cl object layer.
 *
 * \see vvp_ops, lov_ops, lovsub_ops, osc_ops
 */
struct cl_object_operations {
        /**
         * Initialize page slice for this layer. Called top-to-bottom through
         * every object layer when a new cl_page is instantiated. Layer
         * keeping private per-page data, or requiring its own page operations
         * vector should allocate these data here, and attach then to the page
         * by calling cl_page_slice_add(). \a vmpage is locked (in the VM
         * sense). Optional.
         *
         * \retval NULL success.
         *
         * \retval ERR_PTR(errno) failure code.
         *
         * \retval valid-pointer pointer to already existing referenced page
         *         to be used instead of newly created.
         */
        struct cl_page *(*coo_page_init)(const struct lu_env *env,
                                         struct cl_object *obj,
                                         struct cl_page *page,
                                         cfs_page_t *vmpage);
        /**
         * Initialize lock slice for this layer. Called top-to-bottom through
         * every object layer when a new cl_lock is instantiated. Layer
         * keeping private per-lock data, or requiring its own lock operations
         * vector should allocate these data here, and attach then to the lock
         * by calling cl_lock_slice_add(). Mandatory.
         */
        int  (*coo_lock_init)(const struct lu_env *env,
                              struct cl_object *obj, struct cl_lock *lock,
                              const struct cl_io *io);
        /**
         * Initialize io state for a given layer.
         *
         * called top-to-bottom once per io existence to initialize io
         * state. If layer wants to keep some state for this type of io, it
         * has to embed struct cl_io_slice in lu_env::le_ses, and register
         * slice with cl_io_slice_add(). It is guaranteed that all threads
         * participating in this io share the same session.
         */
        int  (*coo_io_init)(const struct lu_env *env,
                            struct cl_object *obj, struct cl_io *io);
        /**
         * Fill portion of \a attr that this layer controls. This method is
         * called top-to-bottom through all object layers.
         *
         * \pre cl_object_header::coh_attr_guard of the top-object is locked.
         *
         * \return   0: to continue
         * \return +ve: to stop iterating through layers (but 0 is returned
         * from enclosing cl_object_attr_get())
         * \return -ve: to signal error
         */
        int (*coo_attr_get)(const struct lu_env *env, struct cl_object *obj,
                            struct cl_attr *attr);
        /**
         * Update attributes.
         *
         * \a valid is a bitmask composed from enum #cl_attr_valid, and
         * indicating what attributes are to be set.
         *
         * \pre cl_object_header::coh_attr_guard of the top-object is locked.
         *
         * \return the same convention as for
         * cl_object_operations::coo_attr_get() is used.
         */
        int (*coo_attr_set)(const struct lu_env *env, struct cl_object *obj,
                            const struct cl_attr *attr, unsigned valid);
        /**
         * Update object configuration. Called top-to-bottom to modify object
         * configuration.
         *
         * XXX error conditions and handling.
         */
        int (*coo_conf_set)(const struct lu_env *env, struct cl_object *obj,
                            const struct cl_object_conf *conf);
        /**
         * Glimpse ast. Executed when glimpse ast arrives for a lock on this
         * object. Layers are supposed to fill parts of \a lvb that will be
         * shipped to the glimpse originator as a glimpse result.
         *
         * \see ccc_object_glimpse(), lovsub_object_glimpse(),
         * \see osc_object_glimpse()
         */
        int (*coo_glimpse)(const struct lu_env *env,
                           const struct cl_object *obj, struct ost_lvb *lvb);
};

/**
 * Extended header for client object.
 */
struct cl_object_header {
        /** Standard lu_object_header. cl_object::co_lu::lo_header points
         * here. */
        struct lu_object_header  coh_lu;
        /** \name locks
         * \todo XXX move locks below to the separate cache-lines, they are
         * mostly useless otherwise.
         */
        /** @{ */
        /** Lock protecting page tree. */
        cfs_spinlock_t           coh_page_guard;
        /** Lock protecting lock list. */
        cfs_spinlock_t           coh_lock_guard;
        /** @} locks */
        /** Radix tree of cl_page's, cached for this object. */
        struct radix_tree_root   coh_tree;
        /** # of pages in radix tree. */
        unsigned long            coh_pages;
        /** List of cl_lock's granted for this object. */
        cfs_list_t               coh_locks;

        /**
         * Parent object. It is assumed that an object has a well-defined
         * parent, but not a well-defined child (there may be multiple
         * sub-objects, for the same top-object). cl_object_header::coh_parent
         * field allows certain code to be written generically, without
         * limiting possible cl_object layouts unduly.
         */
        struct cl_object_header *coh_parent;
        /**
         * Protects consistency between cl_attr of parent object and
         * attributes of sub-objects, that the former is calculated ("merged")
         * from.
         *
         * \todo XXX this can be read/write lock if needed.
         */
        cfs_spinlock_t           coh_attr_guard;
        /**
         * Number of objects above this one: 0 for a top-object, 1 for its
         * sub-object, etc.
         */
        unsigned                 coh_nesting;
};

/**
 * Helper macro: iterate over all layers of the object \a obj, assigning every
 * layer top-to-bottom to \a slice.
 */
#define cl_object_for_each(slice, obj)                                      \
        cfs_list_for_each_entry((slice),                                    \
                                &(obj)->co_lu.lo_header->loh_layers,        \
                                co_lu.lo_linkage)
/**
 * Helper macro: iterate over all layers of the object \a obj, assigning every
 * layer bottom-to-top to \a slice.
 */
#define cl_object_for_each_reverse(slice, obj)                               \
        cfs_list_for_each_entry_reverse((slice),                             \
                                        &(obj)->co_lu.lo_header->loh_layers, \
                                        co_lu.lo_linkage)
/** @} cl_object */

#ifndef pgoff_t
#define pgoff_t unsigned long
#endif

#define CL_PAGE_EOF ((pgoff_t)~0ull)

/** \addtogroup cl_page cl_page
 * @{ */

/** \struct cl_page
 * Layered client page.
 *
 * cl_page: represents a portion of a file, cached in the memory. All pages
 *    of the given file are of the same size, and are kept in the radix tree
 *    hanging off the cl_object. cl_page doesn't fan out, but as sub-objects
 *    of the top-level file object are first class cl_objects, they have their
 *    own radix trees of pages and hence page is implemented as a sequence of
 *    struct cl_pages's, linked into double-linked list through
 *    cl_page::cp_parent and cl_page::cp_child pointers, each residing in the
 *    corresponding radix tree at the corresponding logical offset.
 *
 * cl_page is associated with VM page of the hosting environment (struct
 *    page in Linux kernel, for example), cfs_page_t. It is assumed, that this
 *    association is implemented by one of cl_page layers (top layer in the
 *    current design) that
 *
 *        - intercepts per-VM-page call-backs made by the environment (e.g.,
 *          memory pressure),
 *
 *        - translates state (page flag bits) and locking between lustre and
 *          environment.
 *
 *    The association between cl_page and cfs_page_t is immutable and
 *    established when cl_page is created.
 *
 * cl_page can be "owned" by a particular cl_io (see below), guaranteeing
 *    this io an exclusive access to this page w.r.t. other io attempts and
 *    various events changing page state (such as transfer completion, or
 *    eviction of the page from the memory). Note, that in general cl_io
 *    cannot be identified with a particular thread, and page ownership is not
 *    exactly equal to the current thread holding a lock on the page. Layer
 *    implementing association between cl_page and cfs_page_t has to implement
 *    ownership on top of available synchronization mechanisms.
 *
 *    While lustre client maintains the notion of an page ownership by io,
 *    hosting MM/VM usually has its own page concurrency control
 *    mechanisms. For example, in Linux, page access is synchronized by the
 *    per-page PG_locked bit-lock, and generic kernel code (generic_file_*())
 *    takes care to acquire and release such locks as necessary around the
 *    calls to the file system methods (->readpage(), ->prepare_write(),
 *    ->commit_write(), etc.). This leads to the situation when there are two
 *    different ways to own a page in the client:
 *
 *        - client code explicitly and voluntary owns the page (cl_page_own());
 *
 *        - VM locks a page and then calls the client, that has "to assume"
 *          the ownership from the VM (cl_page_assume()).
 *
 *    Dual methods to release ownership are cl_page_disown() and
 *    cl_page_unassume().
 *
 * cl_page is reference counted (cl_page::cp_ref). When reference counter
 *    drops to 0, the page is returned to the cache, unless it is in
 *    cl_page_state::CPS_FREEING state, in which case it is immediately
 *    destroyed.
 *
 *    The general logic guaranteeing the absence of "existential races" for
 *    pages is the following:
 *
 *        - there are fixed known ways for a thread to obtain a new reference
 *          to a page:
 *
 *            - by doing a lookup in the cl_object radix tree, protected by the
 *              spin-lock;
 *
 *            - by starting from VM-locked cfs_page_t and following some
 *              hosting environment method (e.g., following ->private pointer in
 *              the case of Linux kernel), see cl_vmpage_page();
 *
 *        - when the page enters cl_page_state::CPS_FREEING state, all these
 *          ways are severed with the proper synchronization
 *          (cl_page_delete());
 *
 *        - entry into cl_page_state::CPS_FREEING is serialized by the VM page
 *          lock;
 *
 *        - no new references to the page in cl_page_state::CPS_FREEING state
 *          are allowed (checked in cl_page_get()).
 *
 *    Together this guarantees that when last reference to a
 *    cl_page_state::CPS_FREEING page is released, it is safe to destroy the
 *    page, as neither references to it can be acquired at that point, nor
 *    ones exist.
 *
 * cl_page is a state machine. States are enumerated in enum
 *    cl_page_state. Possible state transitions are enumerated in
 *    cl_page_state_set(). State transition process (i.e., actual changing of
 *    cl_page::cp_state field) is protected by the lock on the underlying VM
 *    page.
 *
 * Linux Kernel implementation.
 *
 *    Binding between cl_page and cfs_page_t (which is a typedef for
 *    struct page) is implemented in the vvp layer. cl_page is attached to the
 *    ->private pointer of the struct page, together with the setting of
 *    PG_private bit in page->flags, and acquiring additional reference on the
 *    struct page (much like struct buffer_head, or any similar file system
 *    private data structures).
 *
 *    PG_locked lock is used to implement both ownership and transfer
 *    synchronization, that is, page is VM-locked in CPS_{OWNED,PAGE{IN,OUT}}
 *    states. No additional references are acquired for the duration of the
 *    transfer.
 *
 * \warning *THIS IS NOT* the behavior expected by the Linux kernel, where
 *          write-out is "protected" by the special PG_writeback bit.
 */

/**
 * States of cl_page. cl_page.c assumes particular order here.
 *
 * The page state machine is rather crude, as it doesn't recognize finer page
 * states like "dirty" or "up to date". This is because such states are not
 * always well defined for the whole stack (see, for example, the
 * implementation of the read-ahead, that hides page up-to-dateness to track
 * cache hits accurately). Such sub-states are maintained by the layers that
 * are interested in them.
 */
enum cl_page_state {
        /**
         * Page is in the cache, un-owned. Page leaves cached state in the
         * following cases:
         *
         *     - [cl_page_state::CPS_OWNED] io comes across the page and
         *     owns it;
         *
         *     - [cl_page_state::CPS_PAGEOUT] page is dirty, the
         *     req-formation engine decides that it wants to include this page
         *     into an cl_req being constructed, and yanks it from the cache;
         *
         *     - [cl_page_state::CPS_FREEING] VM callback is executed to
         *     evict the page form the memory;
         *
         * \invariant cl_page::cp_owner == NULL && cl_page::cp_req == NULL
         */
        CPS_CACHED,
        /**
         * Page is exclusively owned by some cl_io. Page may end up in this
         * state as a result of
         *
         *     - io creating new page and immediately owning it;
         *
         *     - [cl_page_state::CPS_CACHED] io finding existing cached page
         *     and owning it;
         *
         *     - [cl_page_state::CPS_OWNED] io finding existing owned page
         *     and waiting for owner to release the page;
         *
         * Page leaves owned state in the following cases:
         *
         *     - [cl_page_state::CPS_CACHED] io decides to leave the page in
         *     the cache, doing nothing;
         *
         *     - [cl_page_state::CPS_PAGEIN] io starts read transfer for
         *     this page;
         *
         *     - [cl_page_state::CPS_PAGEOUT] io starts immediate write
         *     transfer for this page;
         *
         *     - [cl_page_state::CPS_FREEING] io decides to destroy this
         *     page (e.g., as part of truncate or extent lock cancellation).
         *
         * \invariant cl_page::cp_owner != NULL && cl_page::cp_req == NULL
         */
        CPS_OWNED,
        /**
         * Page is being written out, as a part of a transfer. This state is
         * entered when req-formation logic decided that it wants this page to
         * be sent through the wire _now_. Specifically, it means that once
         * this state is achieved, transfer completion handler (with either
         * success or failure indication) is guaranteed to be executed against
         * this page independently of any locks and any scheduling decisions
         * made by the hosting environment (that effectively means that the
         * page is never put into cl_page_state::CPS_PAGEOUT state "in
         * advance". This property is mentioned, because it is important when
         * reasoning about possible dead-locks in the system). The page can
         * enter this state as a result of
         *
         *     - [cl_page_state::CPS_OWNED] an io requesting an immediate
         *     write-out of this page, or
         *
         *     - [cl_page_state::CPS_CACHED] req-forming engine deciding
         *     that it has enough dirty pages cached to issue a "good"
         *     transfer.
         *
         * The page leaves cl_page_state::CPS_PAGEOUT state when the transfer
         * is completed---it is moved into cl_page_state::CPS_CACHED state.
         *
         * Underlying VM page is locked for the duration of transfer.
         *
         * \invariant: cl_page::cp_owner == NULL && cl_page::cp_req != NULL
         */
        CPS_PAGEOUT,
        /**
         * Page is being read in, as a part of a transfer. This is quite
         * similar to the cl_page_state::CPS_PAGEOUT state, except that
         * read-in is always "immediate"---there is no such thing a sudden
         * construction of read cl_req from cached, presumably not up to date,
         * pages.
         *
         * Underlying VM page is locked for the duration of transfer.
         *
         * \invariant: cl_page::cp_owner == NULL && cl_page::cp_req != NULL
         */
        CPS_PAGEIN,
        /**
         * Page is being destroyed. This state is entered when client decides
         * that page has to be deleted from its host object, as, e.g., a part
         * of truncate.
         *
         * Once this state is reached, there is no way to escape it.
         *
         * \invariant: cl_page::cp_owner == NULL && cl_page::cp_req == NULL
         */
        CPS_FREEING,
        CPS_NR
};

enum cl_page_type {
        /** Host page, the page is from the host inode which the cl_page
         * belongs to. */
        CPT_CACHEABLE = 1,

        /** Transient page, the transient cl_page is used to bind a cl_page
         *  to vmpage which is not belonging to the same object of cl_page.
         *  it is used in DirectIO, lockless IO and liblustre. */
        CPT_TRANSIENT,
};

/**
 * Flags maintained for every cl_page.
 */
enum cl_page_flags {
        /**
         * Set when pagein completes. Used for debugging (read completes at
         * most once for a page).
         */
        CPF_READ_COMPLETED = 1 << 0
};

/**
 * Fields are protected by the lock on cfs_page_t, except for atomics and
 * immutables.
 *
 * \invariant Data type invariants are in cl_page_invariant(). Basically:
 * cl_page::cp_parent and cl_page::cp_child are a well-formed double-linked
 * list, consistent with the parent/child pointers in the cl_page::cp_obj and
 * cl_page::cp_owner (when set).
 */
struct cl_page {
        /** Reference counter. */
        cfs_atomic_t             cp_ref;
        /** An object this page is a part of. Immutable after creation. */
        struct cl_object        *cp_obj;
        /** Logical page index within the object. Immutable after creation. */
        pgoff_t                  cp_index;
        /** List of slices. Immutable after creation. */
        cfs_list_t               cp_layers;
        /** Parent page, NULL for top-level page. Immutable after creation. */
        struct cl_page          *cp_parent;
        /** Lower-layer page. NULL for bottommost page. Immutable after
         * creation. */
        struct cl_page          *cp_child;
        /**
         * Page state. This field is const to avoid accidental update, it is
         * modified only internally within cl_page.c. Protected by a VM lock.
         */
        const enum cl_page_state cp_state;
        /**
         * Linkage of pages within some group. Protected by
         * cl_page::cp_mutex. */
        cfs_list_t               cp_batch;
        /** Mutex serializing membership of a page in a batch. */
        cfs_mutex_t              cp_mutex;
        /** Linkage of pages within cl_req. */
        cfs_list_t               cp_flight;
        /** Transfer error. */
        int                      cp_error;

        /**
         * Page type. Only CPT_TRANSIENT is used so far. Immutable after
         * creation.
         */
        enum cl_page_type        cp_type;

        /**
         * Owning IO in cl_page_state::CPS_OWNED state. Sub-page can be owned
         * by sub-io. Protected by a VM lock.
         */
        struct cl_io            *cp_owner;
        /**
         * Debug information, the task is owning the page.
         */
        cfs_task_t              *cp_task;
        /**
         * Owning IO request in cl_page_state::CPS_PAGEOUT and
         * cl_page_state::CPS_PAGEIN states. This field is maintained only in
         * the top-level pages. Protected by a VM lock.
         */
        struct cl_req           *cp_req;
        /** List of references to this page, for debugging. */
        struct lu_ref            cp_reference;
        /** Link to an object, for debugging. */
        struct lu_ref_link      *cp_obj_ref;
        /** Link to a queue, for debugging. */
        struct lu_ref_link      *cp_queue_ref;
        /** Per-page flags from enum cl_page_flags. Protected by a VM lock. */
        unsigned                 cp_flags;
        /** Assigned if doing a sync_io */
        struct cl_sync_io       *cp_sync_io;
};

/**
 * Per-layer part of cl_page.
 *
 * \see ccc_page, lov_page, osc_page
 */
struct cl_page_slice {
        struct cl_page                  *cpl_page;
        /**
         * Object slice corresponding to this page slice. Immutable after
         * creation.
         */
        struct cl_object                *cpl_obj;
        const struct cl_page_operations *cpl_ops;
        /** Linkage into cl_page::cp_layers. Immutable after creation. */
        cfs_list_t                       cpl_linkage;
};

/**
 * Lock mode. For the client extent locks.
 *
 * \warning: cl_lock_mode_match() assumes particular ordering here.
 * \ingroup cl_lock
 */
enum cl_lock_mode {
        /**
         * Mode of a lock that protects no data, and exists only as a
         * placeholder. This is used for `glimpse' requests. A phantom lock
         * might get promoted to real lock at some point.
         */
        CLM_PHANTOM,
        CLM_READ,
        CLM_WRITE,
        CLM_GROUP
};

/**
 * Requested transfer type.
 * \ingroup cl_req
 */
enum cl_req_type {
        CRT_READ,
        CRT_WRITE,
        CRT_NR
};

/**
 * Per-layer page operations.
 *
 * Methods taking an \a io argument are for the activity happening in the
 * context of given \a io. Page is assumed to be owned by that io, except for
 * the obvious cases (like cl_page_operations::cpo_own()).
 *
 * \see vvp_page_ops, lov_page_ops, osc_page_ops
 */
struct cl_page_operations {
        /**
         * cl_page<->cfs_page_t methods. Only one layer in the stack has to
         * implement these. Current code assumes that this functionality is
         * provided by the topmost layer, see cl_page_disown0() as an example.
         */

        /**
         * \return the underlying VM page. Optional.
         */
        cfs_page_t *(*cpo_vmpage)(const struct lu_env *env,
                                  const struct cl_page_slice *slice);
        /**
         * Called when \a io acquires this page into the exclusive
         * ownership. When this method returns, it is guaranteed that the is
         * not owned by other io, and no transfer is going on against
         * it. Optional.
         *
         * \see cl_page_own()
         * \see vvp_page_own(), lov_page_own()
         */
        int  (*cpo_own)(const struct lu_env *env,
                        const struct cl_page_slice *slice,
                        struct cl_io *io, int nonblock);
        /** Called when ownership it yielded. Optional.
         *
         * \see cl_page_disown()
         * \see vvp_page_disown()
         */
        void (*cpo_disown)(const struct lu_env *env,
                           const struct cl_page_slice *slice, struct cl_io *io);
        /**
         * Called for a page that is already "owned" by \a io from VM point of
         * view. Optional.
         *
         * \see cl_page_assume()
         * \see vvp_page_assume(), lov_page_assume()
         */
        void (*cpo_assume)(const struct lu_env *env,
                           const struct cl_page_slice *slice, struct cl_io *io);
        /** Dual to cl_page_operations::cpo_assume(). Optional. Called
         * bottom-to-top when IO releases a page without actually unlocking
         * it.
         *
         * \see cl_page_unassume()
         * \see vvp_page_unassume()
         */
        void (*cpo_unassume)(const struct lu_env *env,
                             const struct cl_page_slice *slice,
                             struct cl_io *io);
        /**
         * Announces whether the page contains valid data or not by \a uptodate.
         *
         * \see cl_page_export()
         * \see vvp_page_export()
         */
        void  (*cpo_export)(const struct lu_env *env,
                            const struct cl_page_slice *slice, int uptodate);
        /**
         * Unmaps page from the user space (if it is mapped).
         *
         * \see cl_page_unmap()
         * \see vvp_page_unmap()
         */
        int (*cpo_unmap)(const struct lu_env *env,
                         const struct cl_page_slice *slice, struct cl_io *io);
        /**
         * Checks whether underlying VM page is locked (in the suitable
         * sense). Used for assertions.
         *
         * \retval    -EBUSY: page is protected by a lock of a given mode;
         * \retval  -ENODATA: page is not protected by a lock;
         * \retval         0: this layer cannot decide. (Should never happen.)
         */
        int (*cpo_is_vmlocked)(const struct lu_env *env,
                               const struct cl_page_slice *slice);
        /**
         * Page destruction.
         */

        /**
         * Called when page is truncated from the object. Optional.
         *
         * \see cl_page_discard()
         * \see vvp_page_discard(), osc_page_discard()
         */
        void (*cpo_discard)(const struct lu_env *env,
                            const struct cl_page_slice *slice,
                            struct cl_io *io);
        /**
         * Called when page is removed from the cache, and is about to being
         * destroyed. Optional.
         *
         * \see cl_page_delete()
         * \see vvp_page_delete(), osc_page_delete()
         */
        void (*cpo_delete)(const struct lu_env *env,
                           const struct cl_page_slice *slice);
        /** Destructor. Frees resources and slice itself. */
        void (*cpo_fini)(const struct lu_env *env,
                         struct cl_page_slice *slice);

        /**
         * Checks whether the page is protected by a cl_lock. This is a
         * per-layer method, because certain layers have ways to check for the
         * lock much more efficiently than through the generic locks scan, or
         * implement locking mechanisms separate from cl_lock, e.g.,
         * LL_FILE_GROUP_LOCKED in vvp. If \a pending is true, check for locks
         * being canceled, or scheduled for cancellation as soon as the last
         * user goes away, too.
         *
         * \retval    -EBUSY: page is protected by a lock of a given mode;
         * \retval  -ENODATA: page is not protected by a lock;
         * \retval         0: this layer cannot decide.
         *
         * \see cl_page_is_under_lock()
         */
        int (*cpo_is_under_lock)(const struct lu_env *env,
                                 const struct cl_page_slice *slice,
                                 struct cl_io *io);

        /**
         * Optional debugging helper. Prints given page slice.
         *
         * \see cl_page_print()
         */
        int (*cpo_print)(const struct lu_env *env,
                         const struct cl_page_slice *slice,
                         void *cookie, lu_printer_t p);
        /**
         * \name transfer
         *
         * Transfer methods. See comment on cl_req for a description of
         * transfer formation and life-cycle.
         *
         * @{
         */
        /**
         * Request type dependent vector of operations.
         *
         * Transfer operations depend on transfer mode (cl_req_type). To avoid
         * passing transfer mode to each and every of these methods, and to
         * avoid branching on request type inside of the methods, separate
         * methods for cl_req_type:CRT_READ and cl_req_type:CRT_WRITE are
         * provided. That is, method invocation usually looks like
         *
         *         slice->cp_ops.io[req->crq_type].cpo_method(env, slice, ...);
         */
        struct {
                /**
                 * Called when a page is submitted for a transfer as a part of
                 * cl_page_list.
                 *
                 * \return    0         : page is eligible for submission;
                 * \return    -EALREADY : skip this page;
                 * \return    -ve       : error.
                 *
                 * \see cl_page_prep()
                 */
                int  (*cpo_prep)(const struct lu_env *env,
                                 const struct cl_page_slice *slice,
                                 struct cl_io *io);
                /**
                 * Completion handler. This is guaranteed to be eventually
                 * fired after cl_page_operations::cpo_prep() or
                 * cl_page_operations::cpo_make_ready() call.
                 *
                 * This method can be called in a non-blocking context. It is
                 * guaranteed however, that the page involved and its object
                 * are pinned in memory (and, hence, calling cl_page_put() is
                 * safe).
                 *
                 * \see cl_page_completion()
                 */
                void (*cpo_completion)(const struct lu_env *env,
                                       const struct cl_page_slice *slice,
                                       int ioret);
                /**
                 * Called when cached page is about to be added to the
                 * cl_req as a part of req formation.
                 *
                 * \return    0       : proceed with this page;
                 * \return    -EAGAIN : skip this page;
                 * \return    -ve     : error.
                 *
                 * \see cl_page_make_ready()
                 */
                int  (*cpo_make_ready)(const struct lu_env *env,
                                       const struct cl_page_slice *slice);
                /**
                 * Announce that this page is to be written out
                 * opportunistically, that is, page is dirty, it is not
                 * necessary to start write-out transfer right now, but
                 * eventually page has to be written out.
                 *
                 * Main caller of this is the write path (see
                 * vvp_io_commit_write()), using this method to build a
                 * "transfer cache" from which large transfers are then
                 * constructed by the req-formation engine.
                 *
                 * \todo XXX it would make sense to add page-age tracking
                 * semantics here, and to oblige the req-formation engine to
                 * send the page out not later than it is too old.
                 *
                 * \see cl_page_cache_add()
                 */
                int  (*cpo_cache_add)(const struct lu_env *env,
                                      const struct cl_page_slice *slice,
                                      struct cl_io *io);
        } io[CRT_NR];
        /**
         * Tell transfer engine that only [to, from] part of a page should be
         * transmitted.
         *
         * This is used for immediate transfers.
         *
         * \todo XXX this is not very good interface. It would be much better
         * if all transfer parameters were supplied as arguments to
         * cl_io_operations::cio_submit() call, but it is not clear how to do
         * this for page queues.
         *
         * \see cl_page_clip()
         */
        void (*cpo_clip)(const struct lu_env *env,
                         const struct cl_page_slice *slice,
                         int from, int to);
        /**
         * \pre  the page was queued for transferring.
         * \post page is removed from client's pending list, or -EBUSY
         *       is returned if it has already been in transferring.
         *
         * This is one of seldom page operation which is:
         * 0. called from top level;
         * 1. don't have vmpage locked;
         * 2. every layer should synchronize execution of its ->cpo_cancel()
         *    with completion handlers. Osc uses client obd lock for this
         *    purpose. Based on there is no vvp_page_cancel and
         *    lov_page_cancel(), cpo_cancel is defacto protected by client lock.
         *
         * \see osc_page_cancel().
         */
        int (*cpo_cancel)(const struct lu_env *env,
                          const struct cl_page_slice *slice);
        /** @} transfer */
};

/**
 * Helper macro, dumping detailed information about \a page into a log.
 */
#define CL_PAGE_DEBUG(mask, env, page, format, ...)                     \
do {                                                                    \
        LIBCFS_DEBUG_MSG_DATA_DECL(msgdata, mask, NULL);                \
                                                                        \
        if (cfs_cdebug_show(mask, DEBUG_SUBSYSTEM)) {                   \
                cl_page_print(env, &msgdata, lu_cdebug_printer, page);  \
                CDEBUG(mask, format , ## __VA_ARGS__);                  \
        }                                                               \
} while (0)

/**
 * Helper macro, dumping shorter information about \a page into a log.
 */
#define CL_PAGE_HEADER(mask, env, page, format, ...)                          \
do {                                                                          \
        LIBCFS_DEBUG_MSG_DATA_DECL(msgdata, mask, NULL);                      \
                                                                              \
        if (cfs_cdebug_show(mask, DEBUG_SUBSYSTEM)) {                         \
                cl_page_header_print(env, &msgdata, lu_cdebug_printer, page); \
                CDEBUG(mask, format , ## __VA_ARGS__);                        \
        }                                                                     \
} while (0)

/** @} cl_page */

/** \addtogroup cl_lock cl_lock
 * @{ */
/** \struct cl_lock
 *
 * Extent locking on the client.
 *
 * LAYERING
 *
 * The locking model of the new client code is built around
 *
 *        struct cl_lock
 *
 * data-type representing an extent lock on a regular file. cl_lock is a
 * layered object (much like cl_object and cl_page), it consists of a header
 * (struct cl_lock) and a list of layers (struct cl_lock_slice), linked to
 * cl_lock::cll_layers list through cl_lock_slice::cls_linkage.
 *
 * All locks for a given object are linked into cl_object_header::coh_locks
 * list (protected by cl_object_header::coh_lock_guard spin-lock) through
 * cl_lock::cll_linkage. Currently this list is not sorted in any way. We can
 * sort it in starting lock offset, or use altogether different data structure
 * like a tree.
 *
 * Typical cl_lock consists of the two layers:
 *
 *     - vvp_lock (vvp specific data), and
 *     - lov_lock (lov specific data).
 *
 * lov_lock contains an array of sub-locks. Each of these sub-locks is a
 * normal cl_lock: it has a header (struct cl_lock) and a list of layers:
 *
 *     - lovsub_lock, and
 *     - osc_lock
 *
 * Each sub-lock is associated with a cl_object (representing stripe
 * sub-object or the file to which top-level cl_lock is associated to), and is
 * linked into that cl_object::coh_locks. In this respect cl_lock is similar to
 * cl_object (that at lov layer also fans out into multiple sub-objects), and
 * is different from cl_page, that doesn't fan out (there is usually exactly
 * one osc_page for every vvp_page). We shall call vvp-lov portion of the lock
 * a "top-lock" and its lovsub-osc portion a "sub-lock".
 *
 * LIFE CYCLE
 *
 * cl_lock is reference counted. When reference counter drops to 0, lock is
 * placed in the cache, except when lock is in CLS_FREEING state. CLS_FREEING
 * lock is destroyed when last reference is released. Referencing between
 * top-lock and its sub-locks is described in the lov documentation module.
 *
 * STATE MACHINE
 *
 * Also, cl_lock is a state machine. This requires some clarification. One of
 * the goals of client IO re-write was to make IO path non-blocking, or at
 * least to make it easier to make it non-blocking in the future. Here
 * `non-blocking' means that when a system call (read, write, truncate)
 * reaches a situation where it has to wait for a communication with the
 * server, it should --instead of waiting-- remember its current state and
 * switch to some other work.  E.g,. instead of waiting for a lock enqueue,
 * client should proceed doing IO on the next stripe, etc. Obviously this is
 * rather radical redesign, and it is not planned to be fully implemented at
 * this time, instead we are putting some infrastructure in place, that would
 * make it easier to do asynchronous non-blocking IO easier in the
 * future. Specifically, where old locking code goes to sleep (waiting for
 * enqueue, for example), new code returns cl_lock_transition::CLO_WAIT. When
 * enqueue reply comes, its completion handler signals that lock state-machine
 * is ready to transit to the next state. There is some generic code in
 * cl_lock.c that sleeps, waiting for these signals. As a result, for users of
 * this cl_lock.c code, it looks like locking is done in normal blocking
 * fashion, and it the same time it is possible to switch to the non-blocking
 * locking (simply by returning cl_lock_transition::CLO_WAIT from cl_lock.c
 * functions).
 *
 * For a description of state machine states and transitions see enum
 * cl_lock_state.
 *
 * There are two ways to restrict a set of states which lock might move to:
 *
 *     - placing a "hold" on a lock guarantees that lock will not be moved
 *       into cl_lock_state::CLS_FREEING state until hold is released. Hold
 *       can be only acquired on a lock that is not in
 *       cl_lock_state::CLS_FREEING. All holds on a lock are counted in
 *       cl_lock::cll_holds. Hold protects lock from cancellation and
 *       destruction. Requests to cancel and destroy a lock on hold will be
 *       recorded, but only honored when last hold on a lock is released;
 *
 *     - placing a "user" on a lock guarantees that lock will not leave
 *       cl_lock_state::CLS_NEW, cl_lock_state::CLS_QUEUING,
 *       cl_lock_state::CLS_ENQUEUED and cl_lock_state::CLS_HELD set of
 *       states, once it enters this set. That is, if a user is added onto a
 *       lock in a state not from this set, it doesn't immediately enforce
 *       lock to move to this set, but once lock enters this set it will
 *       remain there until all users are removed. Lock users are counted in
 *       cl_lock::cll_users.
 *
 *       User is used to assure that lock is not canceled or destroyed while
 *       it is being enqueued, or actively used by some IO.
 *
 *       Currently, a user always comes with a hold (cl_lock_invariant()
 *       checks that a number of holds is not less than a number of users).
 *
 * CONCURRENCY
 *
 * This is how lock state-machine operates. struct cl_lock contains a mutex
 * cl_lock::cll_guard that protects struct fields.
 *
 *     - mutex is taken, and cl_lock::cll_state is examined.
 *
 *     - for every state there are possible target states where lock can move
 *       into. They are tried in order. Attempts to move into next state are
 *       done by _try() functions in cl_lock.c:cl_{enqueue,unlock,wait}_try().
 *
 *     - if the transition can be performed immediately, state is changed,
 *       and mutex is released.
 *
 *     - if the transition requires blocking, _try() function returns
 *       cl_lock_transition::CLO_WAIT. Caller unlocks mutex and goes to
 *       sleep, waiting for possibility of lock state change. It is woken
 *       up when some event occurs, that makes lock state change possible
 *       (e.g., the reception of the reply from the server), and repeats
 *       the loop.
 *
 * Top-lock and sub-lock has separate mutexes and the latter has to be taken
 * first to avoid dead-lock.
 *
 * To see an example of interaction of all these issues, take a look at the
 * lov_cl.c:lov_lock_enqueue() function. It is called as a part of
 * cl_enqueue_try(), and tries to advance top-lock to ENQUEUED state, by
 * advancing state-machines of its sub-locks (lov_lock_enqueue_one()). Note
 * also, that it uses trylock to grab sub-lock mutex to avoid dead-lock. It
 * also has to handle CEF_ASYNC enqueue, when sub-locks enqueues have to be
 * done in parallel, rather than one after another (this is used for glimpse
 * locks, that cannot dead-lock).
 *
 * INTERFACE AND USAGE
 *
 * struct cl_lock_operations provide a number of call-backs that are invoked
 * when events of interest occurs. Layers can intercept and handle glimpse,
 * blocking, cancel ASTs and a reception of the reply from the server.
 *
 * One important difference with the old client locking model is that new
 * client has a representation for the top-lock, whereas in the old code only
 * sub-locks existed as real data structures and file-level locks are
 * represented by "request sets" that are created and destroyed on each and
 * every lock creation.
 *
 * Top-locks are cached, and can be found in the cache by the system calls. It
 * is possible that top-lock is in cache, but some of its sub-locks were
 * canceled and destroyed. In that case top-lock has to be enqueued again
 * before it can be used.
 *
 * Overall process of the locking during IO operation is as following:
 *
 *     - once parameters for IO are setup in cl_io, cl_io_operations::cio_lock()
 *       is called on each layer. Responsibility of this method is to add locks,
 *       needed by a given layer into cl_io.ci_lockset.
 *
 *     - once locks for all layers were collected, they are sorted to avoid
 *       dead-locks (cl_io_locks_sort()), and enqueued.
 *
 *     - when all locks are acquired, IO is performed;
 *
 *     - locks are released into cache.
 *
 * Striping introduces major additional complexity into locking. The
 * fundamental problem is that it is generally unsafe to actively use (hold)
 * two locks on the different OST servers at the same time, as this introduces
 * inter-server dependency and can lead to cascading evictions.
 *
 * Basic solution is to sub-divide large read/write IOs into smaller pieces so
 * that no multi-stripe locks are taken (note that this design abandons POSIX
 * read/write semantics). Such pieces ideally can be executed concurrently. At
 * the same time, certain types of IO cannot be sub-divived, without
 * sacrificing correctness. This includes:
 *
 *  - O_APPEND write, where [0, EOF] lock has to be taken, to guarantee
 *  atomicity;
 *
 *  - ftruncate(fd, offset), where [offset, EOF] lock has to be taken.
 *
 * Also, in the case of read(fd, buf, count) or write(fd, buf, count), where
 * buf is a part of memory mapped Lustre file, a lock or locks protecting buf
 * has to be held together with the usual lock on [offset, offset + count].
 *
 * As multi-stripe locks have to be allowed, it makes sense to cache them, so
 * that, for example, a sequence of O_APPEND writes can proceed quickly
 * without going down to the individual stripes to do lock matching. On the
 * other hand, multi-stripe locks shouldn't be used by normal read/write
 * calls. To achieve this, every layer can implement ->clo_fits_into() method,
 * that is called by lock matching code (cl_lock_lookup()), and that can be
 * used to selectively disable matching of certain locks for certain IOs. For
 * exmaple, lov layer implements lov_lock_fits_into() that allow multi-stripe
 * locks to be matched only for truncates and O_APPEND writes.
 *
 * Interaction with DLM
 *
 * In the expected setup, cl_lock is ultimately backed up by a collection of
 * DLM locks (struct ldlm_lock). Association between cl_lock and DLM lock is
 * implemented in osc layer, that also matches DLM events (ASTs, cancellation,
 * etc.) into cl_lock_operation calls. See struct osc_lock for a more detailed
 * description of interaction with DLM.
 */

/**
 * Lock description.
 */
struct cl_lock_descr {
        /** Object this lock is granted for. */
        struct cl_object *cld_obj;
        /** Index of the first page protected by this lock. */
        pgoff_t           cld_start;
        /** Index of the last page (inclusive) protected by this lock. */
        pgoff_t           cld_end;
        /** Group ID, for group lock */
        __u64             cld_gid;
        /** Lock mode. */
        enum cl_lock_mode cld_mode;
        /**
         * flags to enqueue lock. A combination of bit-flags from
         * enum cl_enq_flags.
         */
        __u32             cld_enq_flags;
};

#define DDESCR "%s(%d):[%lu, %lu]"
#define PDESCR(descr)                                                   \
        cl_lock_mode_name((descr)->cld_mode), (descr)->cld_mode,        \
        (descr)->cld_start, (descr)->cld_end

const char *cl_lock_mode_name(const enum cl_lock_mode mode);

/**
 * Lock state-machine states.
 *
 * \htmlonly
 * <pre>
 *
 * Possible state transitions:
 *
 *              +------------------>NEW
 *              |                    |
 *              |                    | cl_enqueue_try()
 *              |                    |
 *              |    cl_unuse_try()  V
 *              |  +--------------QUEUING (*)
 *              |  |                 |
 *              |  |                 | cl_enqueue_try()
 *              |  |                 |
 *              |  | cl_unuse_try()  V
 *    sub-lock  |  +-------------ENQUEUED (*)
 *    canceled  |  |                 |
 *              |  |                 | cl_wait_try()
 *              |  |                 |
 *              |  |                (R)
 *              |  |                 |
 *              |  |                 V
 *              |  |                HELD<---------+
 *              |  |                 |            |
 *              |  |                 |            | cl_use_try()
 *              |  |  cl_unuse_try() |            |
 *              |  |                 |            |
 *              |  |                 V         ---+ 
 *              |  +------------>INTRANSIT (D) <--+
 *              |                    |            |
 *              |     cl_unuse_try() |            | cached lock found
 *              |                    |            | cl_use_try()
 *              |                    |            |
 *              |                    V            |
 *              +------------------CACHED---------+
 *                                   |
 *                                  (C)
 *                                   |
 *                                   V
 *                                FREEING
 *
 * Legend:
 *
 *         In states marked with (*) transition to the same state (i.e., a loop
 *         in the diagram) is possible.
 *
 *         (R) is the point where Receive call-back is invoked: it allows layers
 *         to handle arrival of lock reply.
 *
 *         (C) is the point where Cancellation call-back is invoked.
 *
 *         (D) is the transit state which means the lock is changing.
 *
 *         Transition to FREEING state is possible from any other state in the
 *         diagram in case of unrecoverable error.
 * </pre>
 * \endhtmlonly
 *
 * These states are for individual cl_lock object. Top-lock and its sub-locks
 * can be in the different states. Another way to say this is that we have
 * nested state-machines.
 *
 * Separate QUEUING and ENQUEUED states are needed to support non-blocking
 * operation for locks with multiple sub-locks. Imagine lock on a file F, that
 * intersects 3 stripes S0, S1, and S2. To enqueue F client has to send
 * enqueue to S0, wait for its completion, then send enqueue for S1, wait for
 * its completion and at last enqueue lock for S2, and wait for its
 * completion. In that case, top-lock is in QUEUING state while S0, S1 are
 * handled, and is in ENQUEUED state after enqueue to S2 has been sent (note
 * that in this case, sub-locks move from state to state, and top-lock remains
 * in the same state).
 */
enum cl_lock_state {
        /**
         * Lock that wasn't yet enqueued
         */
        CLS_NEW,
        /**
         * Enqueue is in progress, blocking for some intermediate interaction
         * with the other side.
         */
        CLS_QUEUING,
        /**
         * Lock is fully enqueued, waiting for server to reply when it is
         * granted.
         */
        CLS_ENQUEUED,
        /**
         * Lock granted, actively used by some IO.
         */
        CLS_HELD,
        /**
         * This state is used to mark the lock is being used, or unused.
         * We need this state because the lock may have several sublocks,
         * so it's impossible to have an atomic way to bring all sublocks
         * into CLS_HELD state at use case, or all sublocks to CLS_CACHED
         * at unuse case.
         * If a thread is referring to a lock, and it sees the lock is in this
         * state, it must wait for the lock.
         * See state diagram for details.
         */
        CLS_INTRANSIT,
        /**
         * Lock granted, not used.
         */
        CLS_CACHED,
        /**
         * Lock is being destroyed.
         */
        CLS_FREEING,
        CLS_NR
};

enum cl_lock_flags {
        /**
         * lock has been cancelled. This flag is never cleared once set (by
         * cl_lock_cancel0()).
         */
        CLF_CANCELLED  = 1 << 0,
        /** cancellation is pending for this lock. */
        CLF_CANCELPEND = 1 << 1,
        /** destruction is pending for this lock. */
        CLF_DOOMED     = 1 << 2,
        /** from enqueue RPC reply upcall. */
        CLF_FROM_UPCALL= 1 << 3,
};

/**
 * Lock closure.
 *
 * Lock closure is a collection of locks (both top-locks and sub-locks) that
 * might be updated in a result of an operation on a certain lock (which lock
 * this is a closure of).
 *
 * Closures are needed to guarantee dead-lock freedom in the presence of
 *
 *     - nested state-machines (top-lock state-machine composed of sub-lock
 *       state-machines), and
 *
 *     - shared sub-locks.
 *
 * Specifically, many operations, such as lock enqueue, wait, unlock,
 * etc. start from a top-lock, and then operate on a sub-locks of this
 * top-lock, holding a top-lock mutex. When sub-lock state changes as a result
 * of such operation, this change has to be propagated to all top-locks that
 * share this sub-lock. Obviously, no natural lock ordering (e.g.,
 * top-to-bottom or bottom-to-top) captures this scenario, so try-locking has
 * to be used. Lock closure systematizes this try-and-repeat logic.
 */
struct cl_lock_closure {
        /**
         * Lock that is mutexed when closure construction is started. When
         * closure in is `wait' mode (cl_lock_closure::clc_wait), mutex on
         * origin is released before waiting.
         */
        struct cl_lock   *clc_origin;
        /**
         * List of enclosed locks, so far. Locks are linked here through
         * cl_lock::cll_inclosure.
         */
        cfs_list_t        clc_list;
        /**
         * True iff closure is in a `wait' mode. This determines what
         * cl_lock_enclosure() does when a lock L to be added to the closure
         * is currently mutexed by some other thread.
         *
         * If cl_lock_closure::clc_wait is not set, then closure construction
         * fails with CLO_REPEAT immediately.
         *
         * In wait mode, cl_lock_enclosure() waits until next attempt to build
         * a closure might succeed. To this end it releases an origin mutex
         * (cl_lock_closure::clc_origin), that has to be the only lock mutex
         * owned by the current thread, and then waits on L mutex (by grabbing
         * it and immediately releasing), before returning CLO_REPEAT to the
         * caller.
         */
        int               clc_wait;
        /** Number of locks in the closure. */
        int               clc_nr;
};

/**
 * Layered client lock.
 */
struct cl_lock {
        /** Reference counter. */
        cfs_atomic_t          cll_ref;
        /** List of slices. Immutable after creation. */
        cfs_list_t            cll_layers;
        /**
         * Linkage into cl_lock::cll_descr::cld_obj::coh_locks list. Protected
         * by cl_lock::cll_descr::cld_obj::coh_lock_guard.
         */
        cfs_list_t            cll_linkage;
        /**
         * Parameters of this lock. Protected by
         * cl_lock::cll_descr::cld_obj::coh_lock_guard nested within
         * cl_lock::cll_guard. Modified only on lock creation and in
         * cl_lock_modify().
         */
        struct cl_lock_descr  cll_descr;
        /** Protected by cl_lock::cll_guard. */
        enum cl_lock_state    cll_state;
        /** signals state changes. */
        cfs_waitq_t           cll_wq;
        /**
         * Recursive lock, most fields in cl_lock{} are protected by this.
         *
         * Locking rules: this mutex is never held across network
         * communication, except when lock is being canceled.
         *
         * Lock ordering: a mutex of a sub-lock is taken first, then a mutex
         * on a top-lock. Other direction is implemented through a
         * try-lock-repeat loop. Mutices of unrelated locks can be taken only
         * by try-locking.
         *
         * \see osc_lock_enqueue_wait(), lov_lock_cancel(), lov_sublock_wait().
         */
        cfs_mutex_t           cll_guard;
        cfs_task_t           *cll_guarder;
        int                   cll_depth;

        /**
         * the owner for INTRANSIT state
         */
        cfs_task_t           *cll_intransit_owner;
        int                   cll_error;
        /**
         * Number of holds on a lock. A hold prevents a lock from being
         * canceled and destroyed. Protected by cl_lock::cll_guard.
         *
         * \see cl_lock_hold(), cl_lock_unhold(), cl_lock_release()
         */
        int                   cll_holds;
         /**
          * Number of lock users. Valid in cl_lock_state::CLS_HELD state
          * only. Lock user pins lock in CLS_HELD state. Protected by
          * cl_lock::cll_guard.
          *
          * \see cl_wait(), cl_unuse().
          */
        int                   cll_users;
        /**
         * Flag bit-mask. Values from enum cl_lock_flags. Updates are
         * protected by cl_lock::cll_guard.
         */
        unsigned long         cll_flags;
        /**
         * A linkage into a list of locks in a closure.
         *
         * \see cl_lock_closure
         */
        cfs_list_t            cll_inclosure;
        /**
         * Confict lock at queuing time.
         */
        struct cl_lock       *cll_conflict;
        /**
         * A list of references to this lock, for debugging.
         */
        struct lu_ref         cll_reference;
        /**
         * A list of holds on this lock, for debugging.
         */
        struct lu_ref         cll_holders;
        /**
         * A reference for cl_lock::cll_descr::cld_obj. For debugging.
         */
        struct lu_ref_link   *cll_obj_ref;
#ifdef CONFIG_LOCKDEP
        /* "dep_map" name is assumed by lockdep.h macros. */
        struct lockdep_map    dep_map;
#endif
};

/**
 * Per-layer part of cl_lock
 *
 * \see ccc_lock, lov_lock, lovsub_lock, osc_lock
 */
struct cl_lock_slice {
        struct cl_lock                  *cls_lock;
        /** Object slice corresponding to this lock slice. Immutable after
         * creation. */
        struct cl_object                *cls_obj;
        const struct cl_lock_operations *cls_ops;
        /** Linkage into cl_lock::cll_layers. Immutable after creation. */
        cfs_list_t                       cls_linkage;
};

/**
 * Possible (non-error) return values of ->clo_{enqueue,wait,unlock}().
 *
 * NOTE: lov_subresult() depends on ordering here.
 */
enum cl_lock_transition {
        /** operation cannot be completed immediately. Wait for state change. */
        CLO_WAIT        = 1,
        /** operation had to release lock mutex, restart. */
        CLO_REPEAT      = 2,
        /** lower layer re-enqueued. */
        CLO_REENQUEUED  = 3,
};

/**
 *
 * \see vvp_lock_ops, lov_lock_ops, lovsub_lock_ops, osc_lock_ops
 */
struct cl_lock_operations {
        /**
         * \name statemachine
         *
         * State machine transitions. These 3 methods are called to transfer
         * lock from one state to another, as described in the commentary
         * above enum #cl_lock_state.
         *
         * \retval 0          this layer has nothing more to do to before
         *                       transition to the target state happens;
         *
         * \retval CLO_REPEAT method had to release and re-acquire cl_lock
         *                    mutex, repeat invocation of transition method
         *                    across all layers;
         *
         * \retval CLO_WAIT   this layer cannot move to the target state
         *                    immediately, as it has to wait for certain event
         *                    (e.g., the communication with the server). It
         *                    is guaranteed, that when the state transfer
         *                    becomes possible, cl_lock::cll_wq wait-queue
         *                    is signaled. Caller can wait for this event by
         *                    calling cl_lock_state_wait();
         *
         * \retval -ve        failure, abort state transition, move the lock
         *                    into cl_lock_state::CLS_FREEING state, and set
         *                    cl_lock::cll_error.
         *
         * Once all layers voted to agree to transition (by returning 0), lock
         * is moved into corresponding target state. All state transition
         * methods are optional.
         */
        /** @{ */
        /**
         * Attempts to enqueue the lock. Called top-to-bottom.
         *
         * \see ccc_lock_enqueue(), lov_lock_enqueue(), lovsub_lock_enqueue(),
         * \see osc_lock_enqueue()
         */
        int  (*clo_enqueue)(const struct lu_env *env,
                            const struct cl_lock_slice *slice,
                            struct cl_io *io, __u32 enqflags);
        /**
         * Attempts to wait for enqueue result. Called top-to-bottom.
         *
         * \see ccc_lock_wait(), lov_lock_wait(), osc_lock_wait()
         */
        int  (*clo_wait)(const struct lu_env *env,
                         const struct cl_lock_slice *slice);
        /**
         * Attempts to unlock the lock. Called bottom-to-top. In addition to
         * usual return values of lock state-machine methods, this can return
         * -ESTALE to indicate that lock cannot be returned to the cache, and
         * has to be re-initialized.
         * unuse is a one-shot operation, so it must NOT return CLO_WAIT.
         *
         * \see ccc_lock_unuse(), lov_lock_unuse(), osc_lock_unuse()
         */
        int  (*clo_unuse)(const struct lu_env *env,
                          const struct cl_lock_slice *slice);
        /**
         * Notifies layer that cached lock is started being used.
         *
         * \pre lock->cll_state == CLS_CACHED
         *
         * \see lov_lock_use(), osc_lock_use()
         */
        int  (*clo_use)(const struct lu_env *env,
                        const struct cl_lock_slice *slice);
        /** @} statemachine */
        /**
         * A method invoked when lock state is changed (as a result of state
         * transition). This is used, for example, to track when the state of
         * a sub-lock changes, to propagate this change to the corresponding
         * top-lock. Optional
         *
         * \see lovsub_lock_state()
         */
        void (*clo_state)(const struct lu_env *env,
                          const struct cl_lock_slice *slice,
                          enum cl_lock_state st);
        /**
         * Returns true, iff given lock is suitable for the given io, idea
         * being, that there are certain "unsafe" locks, e.g., ones acquired
         * for O_APPEND writes, that we don't want to re-use for a normal
         * write, to avoid the danger of cascading evictions. Optional. Runs
         * under cl_object_header::coh_lock_guard.
         *
         * XXX this should take more information about lock needed by
         * io. Probably lock description or something similar.
         *
         * \see lov_fits_into()
         */
        int (*clo_fits_into)(const struct lu_env *env,
                             const struct cl_lock_slice *slice,
                             const struct cl_lock_descr *need,
                             const struct cl_io *io);
        /**
         * \name ast
         * Asynchronous System Traps. All of then are optional, all are
         * executed bottom-to-top.
         */
        /** @{ */

        /**
         * Cancellation callback. Cancel a lock voluntarily, or under
         * the request of server.
         */
        void (*clo_cancel)(const struct lu_env *env,
                           const struct cl_lock_slice *slice);
        /**
         * Lock weighting ast. Executed to estimate how precious this lock
         * is. The sum of results across all layers is used to determine
         * whether lock worth keeping in cache given present memory usage.
         *
         * \see osc_lock_weigh(), vvp_lock_weigh(), lovsub_lock_weigh().
         */
        unsigned long (*clo_weigh)(const struct lu_env *env,
                                   const struct cl_lock_slice *slice);
        /** @} ast */

        /**
         * \see lovsub_lock_closure()
         */
        int (*clo_closure)(const struct lu_env *env,
                           const struct cl_lock_slice *slice,
                           struct cl_lock_closure *closure);
        /**
         * Executed bottom-to-top when lock description changes (e.g., as a
         * result of server granting more generous lock than was requested).
         *
         * \see lovsub_lock_modify()
         */
        int (*clo_modify)(const struct lu_env *env,
                          const struct cl_lock_slice *slice,
                          const struct cl_lock_descr *updated);
        /**
         * Notifies layers (bottom-to-top) that lock is going to be
         * destroyed. Responsibility of layers is to prevent new references on
         * this lock from being acquired once this method returns.
         *
         * This can be called multiple times due to the races.
         *
         * \see cl_lock_delete()
         * \see osc_lock_delete(), lovsub_lock_delete()
         */
        void (*clo_delete)(const struct lu_env *env,
                           const struct cl_lock_slice *slice);
        /**
         * Destructor. Frees resources and the slice.
         *
         * \see ccc_lock_fini(), lov_lock_fini(), lovsub_lock_fini(),
         * \see osc_lock_fini()
         */
        void (*clo_fini)(const struct lu_env *env, struct cl_lock_slice *slice);
        /**
         * Optional debugging helper. Prints given lock slice.
         */
        int (*clo_print)(const struct lu_env *env,
                         void *cookie, lu_printer_t p,
                         const struct cl_lock_slice *slice);
};

#define CL_LOCK_DEBUG(mask, env, lock, format, ...)                     \
do {                                                                    \
        LIBCFS_DEBUG_MSG_DATA_DECL(msgdata, mask, NULL);                \
                                                                        \
        if (cfs_cdebug_show(mask, DEBUG_SUBSYSTEM)) {                   \
                cl_lock_print(env, &msgdata, lu_cdebug_printer, lock);  \
                CDEBUG(mask, format , ## __VA_ARGS__);                  \
        }                                                               \
} while (0)

/** @} cl_lock */

/** \addtogroup cl_page_list cl_page_list
 * Page list used to perform collective operations on a group of pages.
 *
 * Pages are added to the list one by one. cl_page_list acquires a reference
 * for every page in it. Page list is used to perform collective operations on
 * pages:
 *
 *     - submit pages for an immediate transfer,
 *
 *     - own pages on behalf of certain io (waiting for each page in turn),
 *
 *     - discard pages.
 *
 * When list is finalized, it releases references on all pages it still has.
 *
 * \todo XXX concurrency control.
 *
 * @{
 */
struct cl_page_list {
        unsigned             pl_nr;
        cfs_list_t           pl_pages;
        cfs_task_t          *pl_owner;
};

/** 
 * A 2-queue of pages. A convenience data-type for common use case, 2-queue
 * contains an incoming page list and an outgoing page list.
 */
struct cl_2queue {
        struct cl_page_list c2_qin;
        struct cl_page_list c2_qout;
};

/** @} cl_page_list */

/** \addtogroup cl_io cl_io
 * @{ */
/** \struct cl_io
 * I/O
 *
 * cl_io represents a high level I/O activity like
 * read(2)/write(2)/truncate(2) system call, or cancellation of an extent
 * lock.
 *
 * cl_io is a layered object, much like cl_{object,page,lock} but with one
 * important distinction. We want to minimize number of calls to the allocator
 * in the fast path, e.g., in the case of read(2) when everything is cached:
 * client already owns the lock over region being read, and data are cached
 * due to read-ahead. To avoid allocation of cl_io layers in such situations,
 * per-layer io state is stored in the session, associated with the io, see
 * struct {vvp,lov,osc}_io for example. Sessions allocation is amortized
 * by using free-lists, see cl_env_get().
 *
 * There is a small predefined number of possible io types, enumerated in enum
 * cl_io_type.
 *
 * cl_io is a state machine, that can be advanced concurrently by the multiple
 * threads. It is up to these threads to control the concurrency and,
 * specifically, to detect when io is done, and its state can be safely
 * released.
 *
 * For read/write io overall execution plan is as following:
 *
 *     (0) initialize io state through all layers;
 *
 *     (1) loop: prepare chunk of work to do
 *
 *     (2) call all layers to collect locks they need to process current chunk
 *
 *     (3) sort all locks to avoid dead-locks, and acquire them
 *
 *     (4) process the chunk: call per-page methods
 *         (cl_io_operations::cio_read_page() for read,
 *         cl_io_operations::cio_prepare_write(),
 *         cl_io_operations::cio_commit_write() for write)
 *
 *     (5) release locks
 *
 *     (6) repeat loop.
 *
 * To implement the "parallel IO mode", lov layer creates sub-io's (lazily to
 * address allocation efficiency issues mentioned above), and returns with the
 * special error condition from per-page method when current sub-io has to
 * block. This causes io loop to be repeated, and lov switches to the next
 * sub-io in its cl_io_operations::cio_iter_init() implementation.
 */

/** IO types */
enum cl_io_type {
        /** read system call */
        CIT_READ,
        /** write system call */
        CIT_WRITE,
        /** truncate, utime system calls */
        CIT_SETATTR,
        /**
         * page fault handling
         */
        CIT_FAULT,
        /**
         * Miscellaneous io. This is used for occasional io activity that
         * doesn't fit into other types. Currently this is used for:
         *
         *     - cancellation of an extent lock. This io exists as a context
         *     to write dirty pages from under the lock being canceled back
         *     to the server;
         *
         *     - VM induced page write-out. An io context for writing page out
         *     for memory cleansing;
         *
         *     - glimpse. An io context to acquire glimpse lock.
         *
         *     - grouplock. An io context to acquire group lock.
         *
         * CIT_MISC io is used simply as a context in which locks and pages
         * are manipulated. Such io has no internal "process", that is,
         * cl_io_loop() is never called for it.
         */
        CIT_MISC,
        CIT_OP_NR
};

/**
 * States of cl_io state machine
 */
enum cl_io_state {
        /** Not initialized. */
        CIS_ZERO,
        /** Initialized. */
        CIS_INIT,
        /** IO iteration started. */
        CIS_IT_STARTED,
        /** Locks taken. */
        CIS_LOCKED,
        /** Actual IO is in progress. */
        CIS_IO_GOING,
        /** IO for the current iteration finished. */
        CIS_IO_FINISHED,
        /** Locks released. */
        CIS_UNLOCKED,
        /** Iteration completed. */
        CIS_IT_ENDED,
        /** cl_io finalized. */
        CIS_FINI
};

enum cl_req_priority {
        CRP_NORMAL,
        CRP_CANCEL
};

/**
 * IO state private for a layer.
 *
 * This is usually embedded into layer session data, rather than allocated
 * dynamically.
 *
 * \see vvp_io, lov_io, osc_io, ccc_io
 */
struct cl_io_slice {
        struct cl_io                  *cis_io;
        /** corresponding object slice. Immutable after creation. */
        struct cl_object              *cis_obj;
        /** io operations. Immutable after creation. */
        const struct cl_io_operations *cis_iop;
        /**
         * linkage into a list of all slices for a given cl_io, hanging off
         * cl_io::ci_layers. Immutable after creation.
         */
        cfs_list_t                     cis_linkage;
};


/**
 * Per-layer io operations.
 * \see vvp_io_ops, lov_io_ops, lovsub_io_ops, osc_io_ops
 */
struct cl_io_operations {
        /**
         * Vector of io state transition methods for every io type.
         *
         * \see cl_page_operations::io
         */
        struct {
                /**
                 * Prepare io iteration at a given layer.
                 *
                 * Called top-to-bottom at the beginning of each iteration of
                 * "io loop" (if it makes sense for this type of io). Here
                 * layer selects what work it will do during this iteration.
                 *
                 * \see cl_io_operations::cio_iter_fini()
                 */
                int (*cio_iter_init) (const struct lu_env *env,
                                      const struct cl_io_slice *slice);
                /**
                 * Finalize io iteration.
                 *
                 * Called bottom-to-top at the end of each iteration of "io
                 * loop". Here layers can decide whether IO has to be
                 * continued.
                 *
                 * \see cl_io_operations::cio_iter_init()
                 */
                void (*cio_iter_fini) (const struct lu_env *env,
                                       const struct cl_io_slice *slice);
                /**
                 * Collect locks for the current iteration of io.
                 *
                 * Called top-to-bottom to collect all locks necessary for
                 * this iteration. This methods shouldn't actually enqueue
                 * anything, instead it should post a lock through
                 * cl_io_lock_add(). Once all locks are collected, they are
                 * sorted and enqueued in the proper order.
                 */
                int  (*cio_lock) (const struct lu_env *env,
                                  const struct cl_io_slice *slice);
                /**
                 * Finalize unlocking.
                 *
                 * Called bottom-to-top to finish layer specific unlocking
                 * functionality, after generic code released all locks
                 * acquired by cl_io_operations::cio_lock().
                 */
                void  (*cio_unlock)(const struct lu_env *env,
                                    const struct cl_io_slice *slice);
                /**
                 * Start io iteration.
                 *
                 * Once all locks are acquired, called top-to-bottom to
                 * commence actual IO. In the current implementation,
                 * top-level vvp_io_{read,write}_start() does all the work
                 * synchronously by calling generic_file_*(), so other layers
                 * are called when everything is done.
                 */
                int  (*cio_start)(const struct lu_env *env,
                                  const struct cl_io_slice *slice);
                /**
                 * Called top-to-bottom at the end of io loop. Here layer
                 * might wait for an unfinished asynchronous io.
                 */
                void (*cio_end)  (const struct lu_env *env,
                                  const struct cl_io_slice *slice);
                /**
                 * Called bottom-to-top to notify layers that read/write IO
                 * iteration finished, with \a nob bytes transferred.
                 */
                void (*cio_advance)(const struct lu_env *env,
                                    const struct cl_io_slice *slice,
                                    size_t nob);
                /**
                 * Called once per io, bottom-to-top to release io resources.
                 */
                void (*cio_fini) (const struct lu_env *env,
                                  const struct cl_io_slice *slice);
        } op[CIT_OP_NR];
        struct {
                /**
                 * Submit pages from \a queue->c2_qin for IO, and move
                 * successfully submitted pages into \a queue->c2_qout. Return
                 * non-zero if failed to submit even the single page. If
                 * submission failed after some pages were moved into \a
                 * queue->c2_qout, completion callback with non-zero ioret is
                 * executed on them.
                 */
                int  (*cio_submit)(const struct lu_env *env,
                                   const struct cl_io_slice *slice,
                                   enum cl_req_type crt,
                                   struct cl_2queue *queue,
                                   enum cl_req_priority priority);
        } req_op[CRT_NR];
        /**
         * Read missing page.
         *
         * Called by a top-level cl_io_operations::op[CIT_READ]::cio_start()
         * method, when it hits not-up-to-date page in the range. Optional.
         *
         * \pre io->ci_type == CIT_READ
         */
        int (*cio_read_page)(const struct lu_env *env,
                             const struct cl_io_slice *slice,
                             const struct cl_page_slice *page);
        /**
         * Prepare write of a \a page. Called bottom-to-top by a top-level
         * cl_io_operations::op[CIT_WRITE]::cio_start() to prepare page for
         * get data from user-level buffer.
         *
         * \pre io->ci_type == CIT_WRITE
         *
         * \see vvp_io_prepare_write(), lov_io_prepare_write(),
         * osc_io_prepare_write().
         */
        int (*cio_prepare_write)(const struct lu_env *env,
                                 const struct cl_io_slice *slice,
                                 const struct cl_page_slice *page,
                                 unsigned from, unsigned to);
        /**
         *
         * \pre io->ci_type == CIT_WRITE
         *
         * \see vvp_io_commit_write(), lov_io_commit_write(),
         * osc_io_commit_write().
         */
        int (*cio_commit_write)(const struct lu_env *env,
                                const struct cl_io_slice *slice,
                                const struct cl_page_slice *page,
                                unsigned from, unsigned to);
        /**
         * Optional debugging helper. Print given io slice.
         */
        int (*cio_print)(const struct lu_env *env, void *cookie,
                         lu_printer_t p, const struct cl_io_slice *slice);
};

/**
 * Flags to lock enqueue procedure.
 * \ingroup cl_lock
 */
enum cl_enq_flags {
        /**
         * instruct server to not block, if conflicting lock is found. Instead
         * -EWOULDBLOCK is returned immediately.
         */
        CEF_NONBLOCK     = 0x00000001,
        /**
         * take lock asynchronously (out of order), as it cannot
         * deadlock. This is for LDLM_FL_HAS_INTENT locks used for glimpsing.
         */
        CEF_ASYNC        = 0x00000002,
        /**
         * tell the server to instruct (though a flag in the blocking ast) an
         * owner of the conflicting lock, that it can drop dirty pages
         * protected by this lock, without sending them to the server.
         */
        CEF_DISCARD_DATA = 0x00000004,
        /**
         * tell the sub layers that it must be a `real' lock. This is used for
         * mmapped-buffer locks and glimpse locks that must be never converted
         * into lockless mode.
         *
         * \see vvp_mmap_locks(), cl_glimpse_lock().
         */
        CEF_MUST         = 0x00000008,
        /**
         * tell the sub layers that never request a `real' lock. This flag is
         * not used currently.
         *
         * cl_io::ci_lockreq and CEF_{MUST,NEVER} flags specify lockless
         * conversion policy: ci_lockreq describes generic information of lock
         * requirement for this IO, especially for locks which belong to the
         * object doing IO; however, lock itself may have precise requirements
         * that are described by the enqueue flags.
         */
        CEF_NEVER        = 0x00000010,
        /**
         * for async glimpse lock.
         */
        CEF_AGL          = 0x00000020,
        /**
         * mask of enq_flags.
         */
        CEF_MASK         = 0x0000003f,
};

/**
 * Link between lock and io. Intermediate structure is needed, because the
 * same lock can be part of multiple io's simultaneously.
 */
struct cl_io_lock_link {
        /** linkage into one of cl_lockset lists. */
        cfs_list_t           cill_linkage;
        struct cl_lock_descr cill_descr;
        struct cl_lock      *cill_lock;
        /** optional destructor */
        void               (*cill_fini)(const struct lu_env *env,
                                        struct cl_io_lock_link *link);
};

/**
 * Lock-set represents a collection of locks, that io needs at a
 * time. Generally speaking, client tries to avoid holding multiple locks when
 * possible, because
 *
 *      - holding extent locks over multiple ost's introduces the danger of
 *        "cascading timeouts";
 *
 *      - holding multiple locks over the same ost is still dead-lock prone,
 *        see comment in osc_lock_enqueue(),
 *
 * but there are certain situations where this is unavoidable:
 *
 *      - O_APPEND writes have to take [0, EOF] lock for correctness;
 *
 *      - truncate has to take [new-size, EOF] lock for correctness;
 *
 *      - SNS has to take locks across full stripe for correctness;
 *
 *      - in the case when user level buffer, supplied to {read,write}(file0),
 *        is a part of a memory mapped lustre file, client has to take a dlm
 *        locks on file0, and all files that back up the buffer (or a part of
 *        the buffer, that is being processed in the current chunk, in any
 *        case, there are situations where at least 2 locks are necessary).
 *
 * In such cases we at least try to take locks in the same consistent
 * order. To this end, all locks are first collected, then sorted, and then
 * enqueued.
 */
struct cl_lockset {
        /** locks to be acquired. */
        cfs_list_t  cls_todo;
        /** locks currently being processed. */
        cfs_list_t  cls_curr;
        /** locks acquired. */
        cfs_list_t  cls_done;
};

/**
 * Lock requirements(demand) for IO. It should be cl_io_lock_req,
 * but 'req' is always to be thought as 'request' :-)
 */
enum cl_io_lock_dmd {
        /** Always lock data (e.g., O_APPEND). */
        CILR_MANDATORY = 0,
        /** Layers are free to decide between local and global locking. */
        CILR_MAYBE,
        /** Never lock: there is no cache (e.g., liblustre). */
        CILR_NEVER,
        /** Peek lock: use existing locks, don't queue new ones */
        CILR_PEEK
};

struct cl_io_rw_common {
        loff_t      crw_pos;
        size_t      crw_count;
        int         crw_nonblock;
};


/**
 * State for io.
 *
 * cl_io is shared by all threads participating in this IO (in current
 * implementation only one thread advances IO, but parallel IO design and
 * concurrent copy_*_user() require multiple threads acting on the same IO. It
 * is up to these threads to serialize their activities, including updates to
 * mutable cl_io fields.
 */
struct cl_io {
        /** type of this IO. Immutable after creation. */
        enum cl_io_type                ci_type;
        /** current state of cl_io state machine. */
        enum cl_io_state               ci_state;
        /** main object this io is against. Immutable after creation. */
        struct cl_object              *ci_obj;
        /**
         * Upper layer io, of which this io is a part of. Immutable after
         * creation.
         */
        struct cl_io                  *ci_parent;
        /** List of slices. Immutable after creation. */
        cfs_list_t                     ci_layers;
        /** list of locks (to be) acquired by this io. */
        struct cl_lockset              ci_lockset;
        /** lock requirements, this is just a help info for sublayers. */
        enum cl_io_lock_dmd            ci_lockreq;
        /**
         * This io has held grouplock, to inform sublayers that
         * don't do lockless i/o.
         */
        int                            ci_no_srvlock;
        union {
                struct cl_rd_io {
                        struct cl_io_rw_common rd;
                } ci_rd;
                struct cl_wr_io {
                        struct cl_io_rw_common wr;
                        int                    wr_append;
                } ci_wr;
                struct cl_io_rw_common ci_rw;
                struct cl_setattr_io {
                        struct ost_lvb   sa_attr;
                        unsigned int     sa_valid;
                        struct obd_capa *sa_capa;
                } ci_setattr;
                struct cl_fault_io {
                        /** page index within file. */
                        pgoff_t         ft_index;
                        /** bytes valid byte on a faulted page. */
                        int             ft_nob;
                        /** writable page? for nopage() only */
                        int             ft_writable;
                        /** page of an executable? */
                        int             ft_executable;
                        /** page_mkwrite() */
                        int             ft_mkwrite;
                        /** resulting page */
                        struct cl_page *ft_page;
                } ci_fault;
        } u;
        struct cl_2queue     ci_queue;
        size_t               ci_nob;
        int                  ci_result;
        int                  ci_continue;
        /**
         * Number of pages owned by this IO. For invariant checking.
         */
        unsigned             ci_owned_nr;
};

/** @} cl_io */

/** \addtogroup cl_req cl_req
 * @{ */
/** \struct cl_req
 * Transfer.
 *
 * There are two possible modes of transfer initiation on the client:
 *
 *     - immediate transfer: this is started when a high level io wants a page
 *       or a collection of pages to be transferred right away. Examples:
 *       read-ahead, synchronous read in the case of non-page aligned write,
 *       page write-out as a part of extent lock cancellation, page write-out
 *       as a part of memory cleansing. Immediate transfer can be both
 *       cl_req_type::CRT_READ and cl_req_type::CRT_WRITE;
 *
 *     - opportunistic transfer (cl_req_type::CRT_WRITE only), that happens
 *       when io wants to transfer a page to the server some time later, when
 *       it can be done efficiently. Example: pages dirtied by the write(2)
 *       path.
 *
 * In any case, transfer takes place in the form of a cl_req, which is a
 * representation for a network RPC.
 *
 * Pages queued for an opportunistic transfer are cached until it is decided
 * that efficient RPC can be composed of them. This decision is made by "a
 * req-formation engine", currently implemented as a part of osc
 * layer. Req-formation depends on many factors: the size of the resulting
 * RPC, whether or not multi-object RPCs are supported by the server,
 * max-rpc-in-flight limitations, size of the dirty cache, etc.
 *
 * For the immediate transfer io submits a cl_page_list, that req-formation
 * engine slices into cl_req's, possibly adding cached pages to some of
 * the resulting req's.
 *
 * Whenever a page from cl_page_list is added to a newly constructed req, its
 * cl_page_operations::cpo_prep() layer methods are called. At that moment,
 * page state is atomically changed from cl_page_state::CPS_OWNED to
 * cl_page_state::CPS_PAGEOUT or cl_page_state::CPS_PAGEIN, cl_page::cp_owner
 * is zeroed, and cl_page::cp_req is set to the
 * req. cl_page_operations::cpo_prep() method at the particular layer might
 * return -EALREADY to indicate that it does not need to submit this page
 * at all. This is possible, for example, if page, submitted for read,
 * became up-to-date in the meantime; and for write, the page don't have
 * dirty bit marked. \see cl_io_submit_rw()
 *
 * Whenever a cached page is added to a newly constructed req, its
 * cl_page_operations::cpo_make_ready() layer methods are called. At that
 * moment, page state is atomically changed from cl_page_state::CPS_CACHED to
 * cl_page_state::CPS_PAGEOUT, and cl_page::cp_req is set to
 * req. cl_page_operations::cpo_make_ready() method at the particular layer
 * might return -EAGAIN to indicate that this page is not eligible for the
 * transfer right now.
 *
 * FUTURE
 *
 * Plan is to divide transfers into "priority bands" (indicated when
 * submitting cl_page_list, and queuing a page for the opportunistic transfer)
 * and allow glueing of cached pages to immediate transfers only within single
 * band. This would make high priority transfers (like lock cancellation or
 * memory pressure induced write-out) really high priority.
 *
 */

/**
 * Per-transfer attributes.
 */
struct cl_req_attr {
        /** Generic attributes for the server consumption. */
        struct obdo     *cra_oa;
        /** Capability. */
        struct obd_capa *cra_capa;
};

/**
 * Transfer request operations definable at every layer.
 *
 * Concurrency: transfer formation engine synchronizes calls to all transfer
 * methods.
 */
struct cl_req_operations {
        /**
         * Invoked top-to-bottom by cl_req_prep() when transfer formation is
         * complete (all pages are added).
         *
         * \see osc_req_prep()
         */
        int  (*cro_prep)(const struct lu_env *env,
                         const struct cl_req_slice *slice);
        /**
         * Called top-to-bottom to fill in \a oa fields. This is called twice
         * with different flags, see bug 10150 and osc_build_req().
         *
         * \param obj an object from cl_req which attributes are to be set in
         *            \a oa.
         *
         * \param oa struct obdo where attributes are placed
         *
         * \param flags \a oa fields to be filled.
         */
        void (*cro_attr_set)(const struct lu_env *env,
                             const struct cl_req_slice *slice,
                             const struct cl_object *obj,
                             struct cl_req_attr *attr, obd_valid flags);
        /**
         * Called top-to-bottom from cl_req_completion() to notify layers that
         * transfer completed. Has to free all state allocated by
         * cl_device_operations::cdo_req_init().
         */
        void (*cro_completion)(const struct lu_env *env,
                               const struct cl_req_slice *slice, int ioret);
};

/**
 * A per-object state that (potentially multi-object) transfer request keeps.
 */
struct cl_req_obj {
        /** object itself */
        struct cl_object   *ro_obj;
        /** reference to cl_req_obj::ro_obj. For debugging. */
        struct lu_ref_link *ro_obj_ref;
        /* something else? Number of pages for a given object? */
};

/**
 * Transfer request.
 *
 * Transfer requests are not reference counted, because IO sub-system owns
 * them exclusively and knows when to free them.
 *
 * Life cycle.
 *
 * cl_req is created by cl_req_alloc() that calls
 * cl_device_operations::cdo_req_init() device methods to allocate per-req
 * state in every layer.
 *
 * Then pages are added (cl_req_page_add()), req keeps track of all objects it
 * contains pages for.
 *
 * Once all pages were collected, cl_page_operations::cpo_prep() method is
 * called top-to-bottom. At that point layers can modify req, let it pass, or
 * deny it completely. This is to support things like SNS that have transfer
 * ordering requirements invisible to the individual req-formation engine.
 *
 * On transfer completion (or transfer timeout, or failure to initiate the
 * transfer of an allocated req), cl_req_operations::cro_completion() method
 * is called, after execution of cl_page_operations::cpo_completion() of all
 * req's pages.
 */
struct cl_req {
        enum cl_req_type      crq_type;
        /** A list of pages being transfered */
        cfs_list_t            crq_pages;
        /** Number of pages in cl_req::crq_pages */
        unsigned              crq_nrpages;
        /** An array of objects which pages are in ->crq_pages */
        struct cl_req_obj    *crq_o;
        /** Number of elements in cl_req::crq_objs[] */
        unsigned              crq_nrobjs;
        cfs_list_t            crq_layers;
};

/**
 * Per-layer state for request.
 */
struct cl_req_slice {
        struct cl_req    *crs_req;
        struct cl_device *crs_dev;
        cfs_list_t        crs_linkage;
        const struct cl_req_operations *crs_ops;
};

/* @} cl_req */

/**
 * Stats for a generic cache (similar to inode, lu_object, etc. caches).
 */
struct cache_stats {
        const char    *cs_name;
        /** how many entities were created at all */
        cfs_atomic_t   cs_created;
        /** how many cache lookups were performed */
        cfs_atomic_t   cs_lookup;
        /** how many times cache lookup resulted in a hit */
        cfs_atomic_t   cs_hit;
        /** how many entities are in the cache right now */
        cfs_atomic_t   cs_total;
        /** how many entities in the cache are actively used (and cannot be
         * evicted) right now */
        cfs_atomic_t   cs_busy;
};

/** These are not exported so far */
void cache_stats_init (struct cache_stats *cs, const char *name);
int  cache_stats_print(const struct cache_stats *cs,
                       char *page, int count, int header);

/**
 * Client-side site. This represents particular client stack. "Global"
 * variables should (directly or indirectly) be added here to allow multiple
 * clients to co-exist in the single address space.
 */
struct cl_site {
        struct lu_site        cs_lu;
        /**
         * Statistical counters. Atomics do not scale, something better like
         * per-cpu counters is needed.
         *
         * These are exported as /proc/fs/lustre/llite/.../site
         *
         * When interpreting keep in mind that both sub-locks (and sub-pages)
         * and top-locks (and top-pages) are accounted here.
         */
        struct cache_stats    cs_pages;
        struct cache_stats    cs_locks;
        cfs_atomic_t          cs_pages_state[CPS_NR];
        cfs_atomic_t          cs_locks_state[CLS_NR];
};

int  cl_site_init (struct cl_site *s, struct cl_device *top);
void cl_site_fini (struct cl_site *s);
void cl_stack_fini(const struct lu_env *env, struct cl_device *cl);

/**
 * Output client site statistical counters into a buffer. Suitable for
 * ll_rd_*()-style functions.
 */
int cl_site_stats_print(const struct cl_site *s, char *page, int count);

/**
 * \name helpers
 *
 * Type conversion and accessory functions.
 */
/** @{ */

static inline struct cl_site *lu2cl_site(const struct lu_site *site)
{
        return container_of(site, struct cl_site, cs_lu);
}

static inline int lu_device_is_cl(const struct lu_device *d)
{
        return d->ld_type->ldt_tags & LU_DEVICE_CL;
}

static inline struct cl_device *lu2cl_dev(const struct lu_device *d)
{
        LASSERT(d == NULL || IS_ERR(d) || lu_device_is_cl(d));
        return container_of0(d, struct cl_device, cd_lu_dev);
}

static inline struct lu_device *cl2lu_dev(struct cl_device *d)
{
        return &d->cd_lu_dev;
}

static inline struct cl_object *lu2cl(const struct lu_object *o)
{
        LASSERT(o == NULL || IS_ERR(o) || lu_device_is_cl(o->lo_dev));
        return container_of0(o, struct cl_object, co_lu);
}

static inline const struct cl_object_conf *
lu2cl_conf(const struct lu_object_conf *conf)
{
        return container_of0(conf, struct cl_object_conf, coc_lu);
}

static inline struct cl_object *cl_object_next(const struct cl_object *obj)
{
        return obj ? lu2cl(lu_object_next(&obj->co_lu)) : NULL;
}

static inline struct cl_device *cl_object_device(const struct cl_object *o)
{
        LASSERT(o == NULL || IS_ERR(o) || lu_device_is_cl(o->co_lu.lo_dev));
        return container_of0(o->co_lu.lo_dev, struct cl_device, cd_lu_dev);
}

static inline struct cl_object_header *luh2coh(const struct lu_object_header *h)
{
        return container_of0(h, struct cl_object_header, coh_lu);
}

static inline struct cl_site *cl_object_site(const struct cl_object *obj)
{
        return lu2cl_site(obj->co_lu.lo_dev->ld_site);
}

static inline
struct cl_object_header *cl_object_header(const struct cl_object *obj)
{
        return luh2coh(obj->co_lu.lo_header);
}

static inline int cl_device_init(struct cl_device *d, struct lu_device_type *t)
{
        return lu_device_init(&d->cd_lu_dev, t);
}

static inline void cl_device_fini(struct cl_device *d)
{
        lu_device_fini(&d->cd_lu_dev);
}

void cl_page_slice_add(struct cl_page *page, struct cl_page_slice *slice,
                       struct cl_object *obj,
                       const struct cl_page_operations *ops);
void cl_lock_slice_add(struct cl_lock *lock, struct cl_lock_slice *slice,
                       struct cl_object *obj,
                       const struct cl_lock_operations *ops);
void cl_io_slice_add(struct cl_io *io, struct cl_io_slice *slice,
                     struct cl_object *obj, const struct cl_io_operations *ops);
void cl_req_slice_add(struct cl_req *req, struct cl_req_slice *slice,
                      struct cl_device *dev,
                      const struct cl_req_operations *ops);
/** @} helpers */

/** \defgroup cl_object cl_object
 * @{ */
struct cl_object *cl_object_top (struct cl_object *o);
struct cl_object *cl_object_find(const struct lu_env *env, struct cl_device *cd,
                                 const struct lu_fid *fid,
                                 const struct cl_object_conf *c);

int  cl_object_header_init(struct cl_object_header *h);
void cl_object_header_fini(struct cl_object_header *h);
void cl_object_put        (const struct lu_env *env, struct cl_object *o);
void cl_object_get        (struct cl_object *o);
void cl_object_attr_lock  (struct cl_object *o);
void cl_object_attr_unlock(struct cl_object *o);
int  cl_object_attr_get   (const struct lu_env *env, struct cl_object *obj,
                           struct cl_attr *attr);
int  cl_object_attr_set   (const struct lu_env *env, struct cl_object *obj,
                           const struct cl_attr *attr, unsigned valid);
int  cl_object_glimpse    (const struct lu_env *env, struct cl_object *obj,
                           struct ost_lvb *lvb);
int  cl_conf_set          (const struct lu_env *env, struct cl_object *obj,
                           const struct cl_object_conf *conf);
void cl_object_prune      (const struct lu_env *env, struct cl_object *obj);
void cl_object_kill       (const struct lu_env *env, struct cl_object *obj);
int  cl_object_has_locks  (struct cl_object *obj);

/**
 * Returns true, iff \a o0 and \a o1 are slices of the same object.
 */
static inline int cl_object_same(struct cl_object *o0, struct cl_object *o1)
{
        return cl_object_header(o0) == cl_object_header(o1);
}

/** @} cl_object */

/** \defgroup cl_page cl_page
 * @{ */
enum {
        CLP_GANG_OKAY = 0,
        CLP_GANG_RESCHED,
        CLP_GANG_AGAIN,
        CLP_GANG_ABORT
};

/* callback of cl_page_gang_lookup() */
typedef int   (*cl_page_gang_cb_t)  (const struct lu_env *, struct cl_io *,
                                     struct cl_page *, void *);
int             cl_page_gang_lookup (const struct lu_env *env,
                                     struct cl_object *obj,
                                     struct cl_io *io,
                                     pgoff_t start, pgoff_t end,
                                     cl_page_gang_cb_t cb, void *cbdata);
struct cl_page *cl_page_lookup      (struct cl_object_header *hdr,
                                     pgoff_t index);
struct cl_page *cl_page_find        (const struct lu_env *env,
                                     struct cl_object *obj,
                                     pgoff_t idx, struct page *vmpage,
                                     enum cl_page_type type);
struct cl_page *cl_page_find_sub    (const struct lu_env *env,
                                     struct cl_object *obj,
                                     pgoff_t idx, struct page *vmpage,
                                     struct cl_page *parent);
void            cl_page_get         (struct cl_page *page);
void            cl_page_put         (const struct lu_env *env,
                                     struct cl_page *page);
void            cl_page_print       (const struct lu_env *env, void *cookie,
                                     lu_printer_t printer,
                                     const struct cl_page *pg);
void            cl_page_header_print(const struct lu_env *env, void *cookie,
                                     lu_printer_t printer,
                                     const struct cl_page *pg);
cfs_page_t     *cl_page_vmpage      (const struct lu_env *env,
                                     struct cl_page *page);
struct cl_page *cl_vmpage_page      (cfs_page_t *vmpage, struct cl_object *obj);
struct cl_page *cl_page_top         (struct cl_page *page);

const struct cl_page_slice *cl_page_at(const struct cl_page *page,
                                       const struct lu_device_type *dtype);

/**
 * \name ownership
 *
 * Functions dealing with the ownership of page by io.
 */
/** @{ */

int  cl_page_own        (const struct lu_env *env,
                         struct cl_io *io, struct cl_page *page);
int  cl_page_own_try    (const struct lu_env *env,
                         struct cl_io *io, struct cl_page *page);
void cl_page_assume     (const struct lu_env *env,
                         struct cl_io *io, struct cl_page *page);
void cl_page_unassume   (const struct lu_env *env,
                         struct cl_io *io, struct cl_page *pg);
void cl_page_disown     (const struct lu_env *env,
                         struct cl_io *io, struct cl_page *page);
int  cl_page_is_owned   (const struct cl_page *pg, const struct cl_io *io);

/** @} ownership */

/**
 * \name transfer
 *
 * Functions dealing with the preparation of a page for a transfer, and
 * tracking transfer state.
 */
/** @{ */
int  cl_page_prep       (const struct lu_env *env, struct cl_io *io,
                         struct cl_page *pg, enum cl_req_type crt);
void cl_page_completion (const struct lu_env *env,
                         struct cl_page *pg, enum cl_req_type crt, int ioret);
int  cl_page_make_ready (const struct lu_env *env, struct cl_page *pg,
                         enum cl_req_type crt);
int  cl_page_cache_add  (const struct lu_env *env, struct cl_io *io,
                         struct cl_page *pg, enum cl_req_type crt);
void cl_page_clip       (const struct lu_env *env, struct cl_page *pg,
                         int from, int to);
int  cl_page_cancel     (const struct lu_env *env, struct cl_page *page);

/** @} transfer */


/**
 * \name helper routines
 * Functions to discard, delete and export a cl_page.
 */
/** @{ */
void    cl_page_discard      (const struct lu_env *env, struct cl_io *io,
                              struct cl_page *pg);
void    cl_page_delete       (const struct lu_env *env, struct cl_page *pg);
int     cl_page_unmap        (const struct lu_env *env, struct cl_io *io,
                              struct cl_page *pg);
int     cl_page_is_vmlocked  (const struct lu_env *env,
                              const struct cl_page *pg);
void    cl_page_export       (const struct lu_env *env,
                              struct cl_page *pg, int uptodate);
int     cl_page_is_under_lock(const struct lu_env *env, struct cl_io *io,
                              struct cl_page *page);
loff_t  cl_offset            (const struct cl_object *obj, pgoff_t idx);
pgoff_t cl_index             (const struct cl_object *obj, loff_t offset);
int     cl_page_size         (const struct cl_object *obj);
int     cl_pages_prune       (const struct lu_env *env, struct cl_object *obj);

void cl_lock_print      (const struct lu_env *env, void *cookie,
                         lu_printer_t printer, const struct cl_lock *lock);
void cl_lock_descr_print(const struct lu_env *env, void *cookie,
                         lu_printer_t printer,
                         const struct cl_lock_descr *descr);
/* @} helper */

/** @} cl_page */

/** \defgroup cl_lock cl_lock
 * @{ */

struct cl_lock *cl_lock_hold(const struct lu_env *env, const struct cl_io *io,
                             const struct cl_lock_descr *need,
                             const char *scope, const void *source);
struct cl_lock *cl_lock_peek(const struct lu_env *env, const struct cl_io *io,
                             const struct cl_lock_descr *need,
                             const char *scope, const void *source);
struct cl_lock *cl_lock_request(const struct lu_env *env, struct cl_io *io,
                                const struct cl_lock_descr *need,
                                const char *scope, const void *source);
struct cl_lock *cl_lock_at_page(const struct lu_env *env, struct cl_object *obj,
                                struct cl_page *page, struct cl_lock *except,
                                int pending, int canceld);

const struct cl_lock_slice *cl_lock_at(const struct cl_lock *lock,
                                       const struct lu_device_type *dtype);

void  cl_lock_get       (struct cl_lock *lock);
void  cl_lock_get_trust (struct cl_lock *lock);
void  cl_lock_put       (const struct lu_env *env, struct cl_lock *lock);
void  cl_lock_hold_add  (const struct lu_env *env, struct cl_lock *lock,
                         const char *scope, const void *source);
void  cl_lock_unhold    (const struct lu_env *env, struct cl_lock *lock,
                         const char *scope, const void *source);
void  cl_lock_release   (const struct lu_env *env, struct cl_lock *lock,
                         const char *scope, const void *source);
void  cl_lock_user_add  (const struct lu_env *env, struct cl_lock *lock);
void  cl_lock_user_del  (const struct lu_env *env, struct cl_lock *lock);

enum cl_lock_state cl_lock_intransit(const struct lu_env *env,
                                     struct cl_lock *lock);
void cl_lock_extransit(const struct lu_env *env, struct cl_lock *lock,
                       enum cl_lock_state state);
int cl_lock_is_intransit(struct cl_lock *lock);

int cl_lock_enqueue_wait(const struct lu_env *env, struct cl_lock *lock,
                         int keep_mutex);

/** \name statemachine statemachine
 * Interface to lock state machine consists of 3 parts:
 *
 *     - "try" functions that attempt to effect a state transition. If state
 *     transition is not possible right now (e.g., if it has to wait for some
 *     asynchronous event to occur), these functions return
 *     cl_lock_transition::CLO_WAIT.
 *
 *     - "non-try" functions that implement synchronous blocking interface on
 *     top of non-blocking "try" functions. These functions repeatedly call
 *     corresponding "try" versions, and if state transition is not possible
 *     immediately, wait for lock state change.
 *
 *     - methods from cl_lock_operations, called by "try" functions. Lock can
 *     be advanced to the target state only when all layers voted that they
 *     are ready for this transition. "Try" functions call methods under lock
 *     mutex. If a layer had to release a mutex, it re-acquires it and returns
 *     cl_lock_transition::CLO_REPEAT, causing "try" function to call all
 *     layers again.
 *
 * TRY              NON-TRY      METHOD                            FINAL STATE
 *
 * cl_enqueue_try() cl_enqueue() cl_lock_operations::clo_enqueue() CLS_ENQUEUED
 *
 * cl_wait_try()    cl_wait()    cl_lock_operations::clo_wait()    CLS_HELD
 *
 * cl_unuse_try()   cl_unuse()   cl_lock_operations::clo_unuse()   CLS_CACHED
 *
 * cl_use_try()     NONE         cl_lock_operations::clo_use()     CLS_HELD
 *
 * @{ */

int   cl_enqueue    (const struct lu_env *env, struct cl_lock *lock,
                     struct cl_io *io, __u32 flags);
int   cl_wait       (const struct lu_env *env, struct cl_lock *lock);
void  cl_unuse      (const struct lu_env *env, struct cl_lock *lock);
int   cl_enqueue_try(const struct lu_env *env, struct cl_lock *lock,
                     struct cl_io *io, __u32 flags);
int   cl_unuse_try  (const struct lu_env *env, struct cl_lock *lock);
int   cl_wait_try   (const struct lu_env *env, struct cl_lock *lock);
int   cl_use_try    (const struct lu_env *env, struct cl_lock *lock, int atomic);

/** @} statemachine */

void cl_lock_signal      (const struct lu_env *env, struct cl_lock *lock);
int  cl_lock_state_wait  (const struct lu_env *env, struct cl_lock *lock);
void cl_lock_state_set   (const struct lu_env *env, struct cl_lock *lock,
                          enum cl_lock_state state);
int  cl_queue_match      (const cfs_list_t *queue,
                          const struct cl_lock_descr *need);

void cl_lock_mutex_get  (const struct lu_env *env, struct cl_lock *lock);
int  cl_lock_mutex_try  (const struct lu_env *env, struct cl_lock *lock);
void cl_lock_mutex_put  (const struct lu_env *env, struct cl_lock *lock);
int  cl_lock_is_mutexed (struct cl_lock *lock);
int  cl_lock_nr_mutexed (const struct lu_env *env);
int  cl_lock_page_out   (const struct lu_env *env, struct cl_lock *lock,
                         int discard);
int  cl_lock_ext_match  (const struct cl_lock_descr *has,
                         const struct cl_lock_descr *need);
int  cl_lock_descr_match(const struct cl_lock_descr *has,
                         const struct cl_lock_descr *need);
int  cl_lock_mode_match (enum cl_lock_mode has, enum cl_lock_mode need);
int  cl_lock_modify     (const struct lu_env *env, struct cl_lock *lock,
                         const struct cl_lock_descr *desc);

void cl_lock_closure_init (const struct lu_env *env,
                           struct cl_lock_closure *closure,
                           struct cl_lock *origin, int wait);
void cl_lock_closure_fini (struct cl_lock_closure *closure);
int  cl_lock_closure_build(const struct lu_env *env, struct cl_lock *lock,
                           struct cl_lock_closure *closure);
void cl_lock_disclosure   (const struct lu_env *env,
                           struct cl_lock_closure *closure);
int  cl_lock_enclosure    (const struct lu_env *env, struct cl_lock *lock,
                           struct cl_lock_closure *closure);

void cl_lock_cancel(const struct lu_env *env, struct cl_lock *lock);
void cl_lock_delete(const struct lu_env *env, struct cl_lock *lock);
void cl_lock_error (const struct lu_env *env, struct cl_lock *lock, int error);
void cl_locks_prune(const struct lu_env *env, struct cl_object *obj, int wait);

unsigned long cl_lock_weigh(const struct lu_env *env, struct cl_lock *lock);

/** @} cl_lock */

/** \defgroup cl_io cl_io
 * @{ */

int   cl_io_init         (const struct lu_env *env, struct cl_io *io,
                          enum cl_io_type iot, struct cl_object *obj);
int   cl_io_sub_init     (const struct lu_env *env, struct cl_io *io,
                          enum cl_io_type iot, struct cl_object *obj);
int   cl_io_rw_init      (const struct lu_env *env, struct cl_io *io,
                          enum cl_io_type iot, loff_t pos, size_t count);
int   cl_io_loop         (const struct lu_env *env, struct cl_io *io);

void  cl_io_fini         (const struct lu_env *env, struct cl_io *io);
int   cl_io_iter_init    (const struct lu_env *env, struct cl_io *io);
void  cl_io_iter_fini    (const struct lu_env *env, struct cl_io *io);
int   cl_io_lock         (const struct lu_env *env, struct cl_io *io);
void  cl_io_unlock       (const struct lu_env *env, struct cl_io *io);
int   cl_io_start        (const struct lu_env *env, struct cl_io *io);
void  cl_io_end          (const struct lu_env *env, struct cl_io *io);
int   cl_io_lock_add     (const struct lu_env *env, struct cl_io *io,
                          struct cl_io_lock_link *link);
int   cl_io_lock_alloc_add(const struct lu_env *env, struct cl_io *io,
                           struct cl_lock_descr *descr);
int   cl_io_read_page    (const struct lu_env *env, struct cl_io *io,
                          struct cl_page *page);
int   cl_io_prepare_write(const struct lu_env *env, struct cl_io *io,
                          struct cl_page *page, unsigned from, unsigned to);
int   cl_io_commit_write (const struct lu_env *env, struct cl_io *io,
                          struct cl_page *page, unsigned from, unsigned to);
int   cl_io_submit_rw    (const struct lu_env *env, struct cl_io *io,
                          enum cl_req_type iot, struct cl_2queue *queue,
                          enum cl_req_priority priority);
int   cl_io_submit_sync  (const struct lu_env *env, struct cl_io *io,
                          enum cl_req_type iot, struct cl_2queue *queue,
                          enum cl_req_priority priority, long timeout);
void  cl_io_rw_advance   (const struct lu_env *env, struct cl_io *io,
                          size_t nob);
int   cl_io_cancel       (const struct lu_env *env, struct cl_io *io,
                          struct cl_page_list *queue);
int   cl_io_is_going     (const struct lu_env *env);

/**
 * True, iff \a io is an O_APPEND write(2).
 */
static inline int cl_io_is_append(const struct cl_io *io)
{
        return io->ci_type == CIT_WRITE && io->u.ci_wr.wr_append;
}

/**
 * True, iff \a io is a truncate(2).
 */
static inline int cl_io_is_trunc(const struct cl_io *io)
{
        return io->ci_type == CIT_SETATTR &&
                (io->u.ci_setattr.sa_valid & ATTR_SIZE);
}

struct cl_io *cl_io_top(struct cl_io *io);

void cl_io_print(const struct lu_env *env, void *cookie,
                 lu_printer_t printer, const struct cl_io *io);

#define CL_IO_SLICE_CLEAN(foo_io, base)                                 \
do {                                                                    \
        typeof(foo_io) __foo_io = (foo_io);                             \
                                                                        \
        CLASSERT(offsetof(typeof(*__foo_io), base) == 0);               \
        memset(&__foo_io->base + 1, 0,                                  \
               (sizeof *__foo_io) - sizeof __foo_io->base);             \
} while (0)

/** @} cl_io */

/** \defgroup cl_page_list cl_page_list
 * @{ */

/**
 * Last page in the page list.
 */
static inline struct cl_page *cl_page_list_last(struct cl_page_list *plist)
{
        LASSERT(plist->pl_nr > 0);
        return cfs_list_entry(plist->pl_pages.prev, struct cl_page, cp_batch);
}

/**
 * Iterate over pages in a page list.
 */
#define cl_page_list_for_each(page, list)                               \
        cfs_list_for_each_entry((page), &(list)->pl_pages, cp_batch)

/**
 * Iterate over pages in a page list, taking possible removals into account.
 */
#define cl_page_list_for_each_safe(page, temp, list)                    \
        cfs_list_for_each_entry_safe((page), (temp), &(list)->pl_pages, cp_batch)

void cl_page_list_init   (struct cl_page_list *plist);
void cl_page_list_add    (struct cl_page_list *plist, struct cl_page *page);
void cl_page_list_move   (struct cl_page_list *dst, struct cl_page_list *src,
                          struct cl_page *page);
void cl_page_list_splice (struct cl_page_list *list,
                          struct cl_page_list *head);
void cl_page_list_del    (const struct lu_env *env,
                          struct cl_page_list *plist, struct cl_page *page);
void cl_page_list_disown (const struct lu_env *env,
                          struct cl_io *io, struct cl_page_list *plist);
int  cl_page_list_own    (const struct lu_env *env,
                          struct cl_io *io, struct cl_page_list *plist);
void cl_page_list_assume (const struct lu_env *env,
                          struct cl_io *io, struct cl_page_list *plist);
void cl_page_list_discard(const struct lu_env *env,
                          struct cl_io *io, struct cl_page_list *plist);
int  cl_page_list_unmap  (const struct lu_env *env,
                          struct cl_io *io, struct cl_page_list *plist);
void cl_page_list_fini   (const struct lu_env *env, struct cl_page_list *plist);

void cl_2queue_init     (struct cl_2queue *queue);
void cl_2queue_add      (struct cl_2queue *queue, struct cl_page *page);
void cl_2queue_disown   (const struct lu_env *env,
                         struct cl_io *io, struct cl_2queue *queue);
void cl_2queue_assume   (const struct lu_env *env,
                         struct cl_io *io, struct cl_2queue *queue);
void cl_2queue_discard  (const struct lu_env *env,
                         struct cl_io *io, struct cl_2queue *queue);
void cl_2queue_fini     (const struct lu_env *env, struct cl_2queue *queue);
void cl_2queue_init_page(struct cl_2queue *queue, struct cl_page *page);

/** @} cl_page_list */

/** \defgroup cl_req cl_req
 * @{ */
struct cl_req *cl_req_alloc(const struct lu_env *env, struct cl_page *page,
                            enum cl_req_type crt, int nr_objects);

void cl_req_page_add  (const struct lu_env *env, struct cl_req *req,
                       struct cl_page *page);
void cl_req_page_done (const struct lu_env *env, struct cl_page *page);
int  cl_req_prep      (const struct lu_env *env, struct cl_req *req);
void cl_req_attr_set  (const struct lu_env *env, struct cl_req *req,
                       struct cl_req_attr *attr, obd_valid flags);
void cl_req_completion(const struct lu_env *env, struct cl_req *req, int ioret);

/** \defgroup cl_sync_io cl_sync_io
 * @{ */

/**
 * Anchor for synchronous transfer. This is allocated on a stack by thread
 * doing synchronous transfer, and a pointer to this structure is set up in
 * every page submitted for transfer. Transfer completion routine updates
 * anchor and wakes up waiting thread when transfer is complete.
 */
struct cl_sync_io {
        /** number of pages yet to be transferred. */
        cfs_atomic_t          csi_sync_nr;
        /** completion to be signaled when transfer is complete. */
        cfs_waitq_t          csi_waitq;
        /** error code. */
        int                   csi_sync_rc;
};

void cl_sync_io_init(struct cl_sync_io *anchor, int nrpages);
int  cl_sync_io_wait(const struct lu_env *env, struct cl_io *io,
                     struct cl_page_list *queue, struct cl_sync_io *anchor,
                     long timeout);
void cl_sync_io_note(struct cl_sync_io *anchor, int ioret);

/** @} cl_sync_io */

/** @} cl_req */

/** \defgroup cl_env cl_env
 *
 * lu_env handling for a client.
 *
 * lu_env is an environment within which lustre code executes. Its major part
 * is lu_context---a fast memory allocation mechanism that is used to conserve
 * precious kernel stack space. Originally lu_env was designed for a server,
 * where
 *
 *     - there is a (mostly) fixed number of threads, and
 *
 *     - call chains have no non-lustre portions inserted between lustre code.
 *
 * On a client both these assumtpion fails, because every user thread can
 * potentially execute lustre code as part of a system call, and lustre calls
 * into VFS or MM that call back into lustre.
 *
 * To deal with that, cl_env wrapper functions implement the following
 * optimizations:
 *
 *     - allocation and destruction of environment is amortized by caching no
 *     longer used environments instead of destroying them;
 *
 *     - there is a notion of "current" environment, attached to the kernel
 *     data structure representing current thread Top-level lustre code
 *     allocates an environment and makes it current, then calls into
 *     non-lustre code, that in turn calls lustre back. Low-level lustre
 *     code thus called can fetch environment created by the top-level code
 *     and reuse it, avoiding additional environment allocation.
 *       Right now, three interfaces can attach the cl_env to running thread:
 *       - cl_env_get
 *       - cl_env_implant
 *       - cl_env_reexit(cl_env_reenter had to be called priorly)
 *
 * \see lu_env, lu_context, lu_context_key
 * @{ */

struct cl_env_nest {
        int   cen_refcheck;
        void *cen_cookie;
};

struct lu_env *cl_env_peek       (int *refcheck);
struct lu_env *cl_env_get        (int *refcheck);
struct lu_env *cl_env_alloc      (int *refcheck, __u32 tags);
struct lu_env *cl_env_nested_get (struct cl_env_nest *nest);
void           cl_env_put        (struct lu_env *env, int *refcheck);
void           cl_env_nested_put (struct cl_env_nest *nest, struct lu_env *env);
void          *cl_env_reenter    (void);
void           cl_env_reexit     (void *cookie);
void           cl_env_implant    (struct lu_env *env, int *refcheck);
void           cl_env_unplant    (struct lu_env *env, int *refcheck);
unsigned       cl_env_cache_purge(unsigned nr);

/** @} cl_env */

/*
 * Misc
 */
void cl_attr2lvb(struct ost_lvb *lvb, const struct cl_attr *attr);
void cl_lvb2attr(struct cl_attr *attr, const struct ost_lvb *lvb);

struct cl_device *cl_type_setup(const struct lu_env *env, struct lu_site *site,
                                struct lu_device_type *ldt,
                                struct lu_device *next);
/** @} clio */

#endif /* _LINUX_CL_OBJECT_H */