Commit 34cf78bf authored by Guoju Fang's avatar Guoju Fang Committed by Jens Axboe
Browse files

bcache: fix a lost wake-up problem caused by mca_cannibalize_lock 



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: default avatarGuoju Fang <fangguoju@gmail.com>
Signed-off-by: default avatarColy Li <colyli@suse.de>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent c0e0954e

drivers/md/bcache/bcache.h

+1 −0
Original line number Diff line number Diff line
@@ -582,6 +582,7 @@ 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

drivers/md/bcache/btree.c

+8 −4
Original line number Diff line number Diff line
@@ -910,15 +910,17 @@ 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;

}
@@ -953,10 +955,12 @@ 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,

drivers/md/bcache/super.c

+1 −0
Original line number Diff line number Diff line
@@ -1769,6 +1769,7 @@ 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);

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