From 0b5485391deff35c6633d7cbff5cf3a8229fbe9e Mon Sep 17 00:00:00 2001 From: David Sterba Date: Wed, 1 Mar 2023 21:47:08 +0100 Subject: [PATCH] btrfs: locking: use atomic for DREW lock writers The DREW lock uses percpu variable to track lock counters and for that it needs to allocate the structure. In btrfs_read_tree_root() or btrfs_init_fs_root() this may add another error case or requires the NOFS scope protection. One way is to preallocate the structure as was suggested in https://lore.kernel.org/linux-btrfs/20221214021125.28289-1-robbieko@synology.com/ We may avoid the allocation altogether if we don't use the percpu variables but an atomic for the writer counter. This should not make any difference, the DREW lock is used for truncate and NOCOW writes along with other IO operations. The percpu counter for writers has been there since the original commit 8257b2dc3c1a1057 "Btrfs: introduce btrfs_{start, end}_nocow_write() for each subvolume". The reason could be to avoid hammering the same cacheline from all the readers but then the writers do that anyway. Signed-off-by: David Sterba --- fs/btrfs/disk-io.c | 12 +----------- fs/btrfs/locking.c | 25 ++++++------------------- fs/btrfs/locking.h | 5 ++--- 3 files changed, 9 insertions(+), 33 deletions(-) diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c index 991ff26..1b1b9e8 100644 --- a/fs/btrfs/disk-io.c +++ b/fs/btrfs/disk-io.c @@ -1341,17 +1341,8 @@ struct btrfs_root *btrfs_read_tree_root(struct btrfs_root *tree_root, static int btrfs_init_fs_root(struct btrfs_root *root, dev_t anon_dev) { int ret; - unsigned int nofs_flag; - /* - * We might be called under a transaction (e.g. indirect backref - * resolution) which could deadlock if it triggers memory reclaim - */ - nofs_flag = memalloc_nofs_save(); - ret = btrfs_drew_lock_init(&root->snapshot_lock); - memalloc_nofs_restore(nofs_flag); - if (ret) - goto fail; + btrfs_drew_lock_init(&root->snapshot_lock); if (root->root_key.objectid != BTRFS_TREE_LOG_OBJECTID && !btrfs_is_data_reloc_root(root)) { @@ -2065,7 +2056,6 @@ void btrfs_put_root(struct btrfs_root *root) WARN_ON(test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state)); if (root->anon_dev) free_anon_bdev(root->anon_dev); - btrfs_drew_lock_destroy(&root->snapshot_lock); free_root_extent_buffers(root); #ifdef CONFIG_BTRFS_DEBUG spin_lock(&root->fs_info->fs_roots_radix_lock); diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c index 870528d..3a496b0d 100644 --- a/fs/btrfs/locking.c +++ b/fs/btrfs/locking.c @@ -325,24 +325,12 @@ struct extent_buffer *btrfs_try_read_lock_root_node(struct btrfs_root *root) * acquire the lock. */ -int btrfs_drew_lock_init(struct btrfs_drew_lock *lock) +void btrfs_drew_lock_init(struct btrfs_drew_lock *lock) { - int ret; - - ret = percpu_counter_init(&lock->writers, 0, GFP_KERNEL); - if (ret) - return ret; - atomic_set(&lock->readers, 0); + atomic_set(&lock->writers, 0); init_waitqueue_head(&lock->pending_readers); init_waitqueue_head(&lock->pending_writers); - - return 0; -} - -void btrfs_drew_lock_destroy(struct btrfs_drew_lock *lock) -{ - percpu_counter_destroy(&lock->writers); } /* Return true if acquisition is successful, false otherwise */ @@ -351,10 +339,10 @@ bool btrfs_drew_try_write_lock(struct btrfs_drew_lock *lock) if (atomic_read(&lock->readers)) return false; - percpu_counter_inc(&lock->writers); + atomic_inc(&lock->writers); /* Ensure writers count is updated before we check for pending readers */ - smp_mb(); + smp_mb__after_atomic(); if (atomic_read(&lock->readers)) { btrfs_drew_write_unlock(lock); return false; @@ -374,7 +362,7 @@ void btrfs_drew_write_lock(struct btrfs_drew_lock *lock) void btrfs_drew_write_unlock(struct btrfs_drew_lock *lock) { - percpu_counter_dec(&lock->writers); + atomic_dec(&lock->writers); cond_wake_up(&lock->pending_readers); } @@ -390,8 +378,7 @@ void btrfs_drew_read_lock(struct btrfs_drew_lock *lock) */ smp_mb__after_atomic(); - wait_event(lock->pending_readers, - percpu_counter_sum(&lock->writers) == 0); + wait_event(lock->pending_readers, atomic_read(&lock->writers) == 0); } void btrfs_drew_read_unlock(struct btrfs_drew_lock *lock) diff --git a/fs/btrfs/locking.h b/fs/btrfs/locking.h index 11c2269..edb9b4a 100644 --- a/fs/btrfs/locking.h +++ b/fs/btrfs/locking.h @@ -195,13 +195,12 @@ static inline void btrfs_tree_unlock_rw(struct extent_buffer *eb, int rw) struct btrfs_drew_lock { atomic_t readers; - struct percpu_counter writers; + atomic_t writers; wait_queue_head_t pending_writers; wait_queue_head_t pending_readers; }; -int btrfs_drew_lock_init(struct btrfs_drew_lock *lock); -void btrfs_drew_lock_destroy(struct btrfs_drew_lock *lock); +void btrfs_drew_lock_init(struct btrfs_drew_lock *lock); void btrfs_drew_write_lock(struct btrfs_drew_lock *lock); bool btrfs_drew_try_write_lock(struct btrfs_drew_lock *lock); void btrfs_drew_write_unlock(struct btrfs_drew_lock *lock); -- 2.7.4