LDLM Lock Descriptor
^^^^^^^^^^^^^^^^^^^^
[[struct-ldlm-lock-desc]]

The lock descriptor conveys the specific details about a particular
lock being requested or granted. It appears in
<<struct-ldlm-request>>.

----
struct ldlm_lock_desc {
        struct ldlm_resource_desc l_resource;
        ldlm_mode_t l_req_mode;
        ldlm_mode_t l_granted_mode;
        ldlm_wire_policy_data_t l_policy_data;
};
----

The 'l_resource' field identifies the resource upon which the lock is
being requested or granted. See the description of
<<struct-ldlm-resource-desc>>.

The 'l_req_mode' and 'l_granted_mode' fields give the kind of lock
being requested and the kind of lock that has been granted. The field
values are:

----
typedef enum {
        LCK_EX      = 1, /* exclusive */
        LCK_PW      = 2, /* privileged write */
        LCK_PR      = 4, /* privileged read */
        LCK_CW      = 8, /* concurrent write */
        LCK_CR      = 16, /* concurrent read */
        LCK_NL      = 32, /* */
        LCK_GROUP   = 64, /* */
        LCK_COS     = 128, /* */
} ldlm_mode_t;
----

Despite the fact that the lock modes are not overlapping, these lock
modes are exclusive.  In addition the mode value 0 is the MINMODE,
i.e. no lock at all.

In a request 'l_req_mode' is the value actually being requested and
'l_granted_mode' is the value that currently is in place on for the
requester. In a reply the 'l_req_mode' may be modified if more or
fewer privileges were granted than requested, and the
'l_granted_mode' is what has, in fact, been granted.

The 'l_policy_data' field gives the kind of resource being
requested/granted. It is a union of these struct definitions:
[[ldlm-wire-policy-data-t]]

----
typedef union {
        struct ldlm_extent l_extent;
        struct ldlm_flock_wire l_flock;
        struct ldlm_inodebits l_inodebits;
} ldlm_wire_policy_data_t;
----

----
struct ldlm_extent {
        __u64 start;
        __u64 end;
        __u64 gid;
};
struct ldlm_flock_wire {
        __u64 lfw_start;
        __u64 lfw_end;
        __u64 lfw_owner;
        __u32 lfw_padding;
        __u32 lfw_pid;
};
struct ldlm_inodebits {
        __u64 bits;
};
----

Thus the lock may be on an 'extent', a contiguous sequence of bytes
in a regular file; an 'flock wire', whatever to heck that is; or a
portion of an inode. For a "plain" lock (or one with no type at all)
the 'l_policy_data' field has zero length.