This is a note to let you know that I've just added the patch titled bcache: fix a lost wake-up problem caused by mca_cannibalize_lock to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: bcache-fix-a-lost-wake-up-problem-caused-by-mca_cann.patch and it can be found in the queue-4.9 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 8f0edc56edde0a3eb7a9475e25408d5a63a50f34 Author: Guoju Fang <fangguoju@xxxxxxxxx> Date: Wed Nov 13 16:03:16 2019 +0800 bcache: fix a lost wake-up problem caused by mca_cannibalize_lock [ Upstream commit 34cf78bf34d48dddddfeeadb44f9841d7864997a ] This patch fix a lost wake-up problem caused by the race between mca_cannibalize_lock and bch_cannibalize_unlock. Consider two processes, A and B. Process A is executing mca_cannibalize_lock, while process B takes c->btree_cache_alloc_lock and is executing bch_cannibalize_unlock. The problem happens that after process A executes cmpxchg and will execute prepare_to_wait. In this timeslice process B executes wake_up, but after that process A executes prepare_to_wait and set the state to TASK_INTERRUPTIBLE. Then process A goes to sleep but no one will wake up it. This problem may cause bcache device to dead. Signed-off-by: Guoju Fang <fangguoju@xxxxxxxxx> Signed-off-by: Coly Li <colyli@xxxxxxx> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h index 7fe7df56fa334..f0939fc1cfe55 100644 --- a/drivers/md/bcache/bcache.h +++ b/drivers/md/bcache/bcache.h @@ -547,6 +547,7 @@ struct cache_set { */ wait_queue_head_t btree_cache_wait; struct task_struct *btree_cache_alloc_lock; + spinlock_t btree_cannibalize_lock; /* * When we free a btree node, we increment the gen of the bucket the diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c index 764d519a7f1c6..26e56a9952d09 100644 --- a/drivers/md/bcache/btree.c +++ b/drivers/md/bcache/btree.c @@ -836,15 +836,17 @@ out: static int mca_cannibalize_lock(struct cache_set *c, struct btree_op *op) { - struct task_struct *old; - - old = cmpxchg(&c->btree_cache_alloc_lock, NULL, current); - if (old && old != current) { + spin_lock(&c->btree_cannibalize_lock); + if (likely(c->btree_cache_alloc_lock == NULL)) { + c->btree_cache_alloc_lock = current; + } else if (c->btree_cache_alloc_lock != current) { if (op) prepare_to_wait(&c->btree_cache_wait, &op->wait, TASK_UNINTERRUPTIBLE); + spin_unlock(&c->btree_cannibalize_lock); return -EINTR; } + spin_unlock(&c->btree_cannibalize_lock); return 0; } @@ -879,10 +881,12 @@ static struct btree *mca_cannibalize(struct cache_set *c, struct btree_op *op, */ static void bch_cannibalize_unlock(struct cache_set *c) { + spin_lock(&c->btree_cannibalize_lock); if (c->btree_cache_alloc_lock == current) { c->btree_cache_alloc_lock = NULL; wake_up(&c->btree_cache_wait); } + spin_unlock(&c->btree_cannibalize_lock); } static struct btree *mca_alloc(struct cache_set *c, struct btree_op *op, diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c index 95e9a33de06a2..263c0d987929e 100644 --- a/drivers/md/bcache/super.c +++ b/drivers/md/bcache/super.c @@ -1510,6 +1510,7 @@ struct cache_set *bch_cache_set_alloc(struct cache_sb *sb) sema_init(&c->sb_write_mutex, 1); mutex_init(&c->bucket_lock); init_waitqueue_head(&c->btree_cache_wait); + spin_lock_init(&c->btree_cannibalize_lock); init_waitqueue_head(&c->bucket_wait); init_waitqueue_head(&c->gc_wait); sema_init(&c->uuid_write_mutex, 1);