From e5914a61ac7705f5768c9c75094a456dafefb04f Mon Sep 17 00:00:00 2001 From: NeilBrown Date: Fri, 19 Jul 2019 14:57:16 -0400 Subject: [PATCH] LU-12460 llite: replace lli_trunc_sem lli_trunc_sem can lead to a deadlock. vvp_io_read_start takes lli_trunc_sem, and can take mmap sem in the direct i/o case, via generic_file_read_iter->ll_direct_IO->get_user_pages_unlocked vvp_io_fault_start is called with mmap_sem held (taken in the kernel page fault code), and takes lli_trunc_sem. These aren't necessarily the same mmap_sem, but can be if you mmap a lustre file, then read into that mapped memory from the file. These are both 'down_read' calls on lli_trunc_sem so they don't directly conflict, but if vvp_io_setattr_start() is called to truncate the file between these, it does 'down_write' on lli_trunc_sem. As semaphores are queued, this down_write blocks subsequent reads. This means if the page fault has taken the mmap_sem, but not yet the lli_trunc_sem in vvp_io_fault_start, it will wait behind the lli_trunc_sem down_write from vvp_io_setattr_start. At the same time, vvp_io_read_start is holding the lli_trunc_sem and waiting for the mmap_sem, which will not be released because vvp_io_fault_start cannot get the lli_trunc_sem because the setattr 'down_write' operation is queued in front of it. Solve this by replacing with a hand-coded semaphore, using atomic counters and wait_var_event(). This allows a special down_read_nowait which ignores waiting down_write operations. This combined with waking up all waiters at once guarantees that down_read_nowait can always 'join' another down_read, guaranteeing our ability to take the semaphore twice for read and avoiding the deadlock. I'd like there to be a better way to fix this, but I haven't found it yet. Signed-off-by: NeilBrown Signed-off-by: Patrick Farrell Change-Id: Ibd3abf4df1f1f6f45e440733a364999bd608b191 Reviewed-on: https://review.whamcloud.com/35271 Reviewed-by: Neil Brown Tested-by: jenkins Tested-by: Maloo Reviewed-by: Shaun Tancheff Reviewed-by: Oleg Drokin --- lustre/llite/llite_internal.h | 100 ++++++++++++++++++++++++++++++++++++++---- lustre/llite/llite_lib.c | 2 +- lustre/llite/vvp_io.c | 14 +++--- 3 files changed, 100 insertions(+), 16 deletions(-) diff --git a/lustre/llite/llite_internal.h b/lustre/llite/llite_internal.h index 1649f11..0fb485f 100644 --- a/lustre/llite/llite_internal.h +++ b/lustre/llite/llite_internal.h @@ -103,6 +103,16 @@ struct ll_grouplock { unsigned long lg_gid; }; +/* See comment on trunc_sem_down_read_nowait */ +struct ll_trunc_sem { + /* when positive, this is a count of readers, when -1, it indicates + * the semaphore is held for write, and 0 is unlocked + */ + atomic_t ll_trunc_readers; + /* this tracks a count of waiting writers */ + atomic_t ll_trunc_waiters; +}; + struct ll_inode_info { __u32 lli_inode_magic; spinlock_t lli_lock; @@ -171,14 +181,7 @@ struct ll_inode_info { struct { struct mutex lli_size_mutex; char *lli_symlink_name; - /* - * struct rw_semaphore { - * signed long count; // align d.d_def_acl - * spinlock_t wait_lock; // align d.d_sa_lock - * struct list_head wait_list; - * } - */ - struct rw_semaphore lli_trunc_sem; + struct ll_trunc_sem lli_trunc_sem; struct range_lock_tree lli_write_tree; struct rw_semaphore lli_glimpse_sem; @@ -254,6 +257,87 @@ struct ll_inode_info { struct list_head lli_xattrs; /* ll_xattr_entry->xe_list */ }; +static inline void ll_trunc_sem_init(struct ll_trunc_sem *sem) +{ + atomic_set(&sem->ll_trunc_readers, 0); + atomic_set(&sem->ll_trunc_waiters, 0); +} + +/* This version of down read ignores waiting writers, meaning if the semaphore + * is already held for read, this down_read will 'join' that reader and also + * take the semaphore. + * + * This lets us avoid an unusual deadlock. + * + * We must take lli_trunc_sem in read mode on entry in to various i/o paths + * in Lustre, in order to exclude truncates. Some of these paths then need to + * take the mmap_sem, while still holding the trunc_sem. The problem is that + * page faults hold the mmap_sem when calling in to Lustre, and then must also + * take the trunc_sem to exclude truncate. + * + * This means the locking order for trunc_sem and mmap_sem is sometimes AB, + * sometimes BA. This is almost OK because in both cases, we take the trunc + * sem for read, so it doesn't block. + * + * However, if a write mode user (truncate, a setattr op) arrives in the + * middle of this, the second reader on the truncate_sem will wait behind that + * writer. + * + * So we have, on our truncate sem, in order (where 'reader' and 'writer' refer + * to the mode in which they take the semaphore): + * reader (holding mmap_sem, needs truncate_sem) + * writer + * reader (holding truncate sem, waiting for mmap_sem) + * + * And so the readers deadlock. + * + * The solution is this modified semaphore, where this down_read ignores + * waiting write operations, and all waiters are woken up at once, so readers + * using down_read_nowait cannot get stuck behind waiting writers, regardless + * of the order they arrived in. + * + * down_read_nowait is only used in the page fault case, where we already hold + * the mmap_sem. This is because otherwise repeated read and write operations + * (which take the truncate sem) could prevent a truncate from ever starting. + * This could still happen with page faults, but without an even more complex + * mechanism, this is unavoidable. + * + * LU-12460 + */ +static inline void trunc_sem_down_read_nowait(struct ll_trunc_sem *sem) +{ + wait_var_event(&sem->ll_trunc_readers, + atomic_inc_unless_negative(&sem->ll_trunc_readers)); +} + +static inline void trunc_sem_down_read(struct ll_trunc_sem *sem) +{ + wait_var_event(&sem->ll_trunc_readers, + atomic_read(&sem->ll_trunc_waiters) == 0 && + atomic_inc_unless_negative(&sem->ll_trunc_readers)); +} + +static inline void trunc_sem_up_read(struct ll_trunc_sem *sem) +{ + if (atomic_dec_return(&sem->ll_trunc_readers) == 0 && + atomic_read(&sem->ll_trunc_waiters)) + wake_up_var(&sem->ll_trunc_readers); +} + +static inline void trunc_sem_down_write(struct ll_trunc_sem *sem) +{ + atomic_inc(&sem->ll_trunc_waiters); + wait_var_event(&sem->ll_trunc_readers, + atomic_cmpxchg(&sem->ll_trunc_readers, 0, -1) == 0); + atomic_dec(&sem->ll_trunc_waiters); +} + +static inline void trunc_sem_up_write(struct ll_trunc_sem *sem) +{ + atomic_set(&sem->ll_trunc_readers, 0); + wake_up_var(&sem->ll_trunc_readers); +} + static inline __u32 ll_layout_version_get(struct ll_inode_info *lli) { __u32 gen; diff --git a/lustre/llite/llite_lib.c b/lustre/llite/llite_lib.c index 69a50629..6bd2032 100644 --- a/lustre/llite/llite_lib.c +++ b/lustre/llite/llite_lib.c @@ -992,7 +992,7 @@ void ll_lli_init(struct ll_inode_info *lli) } else { mutex_init(&lli->lli_size_mutex); lli->lli_symlink_name = NULL; - init_rwsem(&lli->lli_trunc_sem); + ll_trunc_sem_init(&lli->lli_trunc_sem); range_lock_tree_init(&lli->lli_write_tree); init_rwsem(&lli->lli_glimpse_sem); lli->lli_glimpse_time = ktime_set(0, 0); diff --git a/lustre/llite/vvp_io.c b/lustre/llite/vvp_io.c index 9ae162f..f0ce954 100644 --- a/lustre/llite/vvp_io.c +++ b/lustre/llite/vvp_io.c @@ -719,7 +719,7 @@ static int vvp_io_setattr_start(const struct lu_env *env, struct ll_inode_info *lli = ll_i2info(inode); if (cl_io_is_trunc(io)) { - down_write(&lli->lli_trunc_sem); + trunc_sem_down_write(&lli->lli_trunc_sem); inode_lock(inode); inode_dio_wait(inode); } else { @@ -745,7 +745,7 @@ static void vvp_io_setattr_end(const struct lu_env *env, vvp_do_vmtruncate(inode, io->u.ci_setattr.sa_attr.lvb_size); inode_dio_write_done(inode); inode_unlock(inode); - up_write(&lli->lli_trunc_sem); + trunc_sem_up_write(&lli->lli_trunc_sem); } else { inode_unlock(inode); } @@ -788,7 +788,7 @@ static int vvp_io_read_start(const struct lu_env *env, pos, pos + cnt); if (vio->vui_io_subtype == IO_NORMAL) - down_read(&lli->lli_trunc_sem); + trunc_sem_down_read(&lli->lli_trunc_sem); if (io->ci_async_readahead) { file_accessed(file); @@ -1186,7 +1186,7 @@ static int vvp_io_write_start(const struct lu_env *env, ENTRY; if (vio->vui_io_subtype == IO_NORMAL) - down_read(&lli->lli_trunc_sem); + trunc_sem_down_read(&lli->lli_trunc_sem); if (!can_populate_pages(env, io, inode)) RETURN(0); @@ -1306,7 +1306,7 @@ static void vvp_io_rw_end(const struct lu_env *env, struct ll_inode_info *lli = ll_i2info(inode); if (vio->vui_io_subtype == IO_NORMAL) - up_read(&lli->lli_trunc_sem); + trunc_sem_up_read(&lli->lli_trunc_sem); } static int vvp_io_kernel_fault(struct vvp_fault_io *cfio) @@ -1371,7 +1371,7 @@ static int vvp_io_fault_start(const struct lu_env *env, pgoff_t last_index; ENTRY; - down_read(&lli->lli_trunc_sem); + trunc_sem_down_read_nowait(&lli->lli_trunc_sem); /* offset of the last byte on the page */ offset = cl_offset(obj, fio->ft_index + 1) - 1; @@ -1521,7 +1521,7 @@ static void vvp_io_fault_end(const struct lu_env *env, CLOBINVRNT(env, ios->cis_io->ci_obj, vvp_object_invariant(ios->cis_io->ci_obj)); - up_read(&lli->lli_trunc_sem); + trunc_sem_up_read(&lli->lli_trunc_sem); } static int vvp_io_fsync_start(const struct lu_env *env, -- 1.8.3.1