[PATCH AUTOSEL 4.4 10/64] bcache: fix a lost wake-up problem caused by mca_cannibalize_lock

Sasha Levin
2020-09-18T02:23:22Z

From: Guoju Fang <fangguoju@gmail.com>

[ 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@gmail.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/md/bcache/bcache.h | 1 +
drivers/md/bcache/btree.c | 12 ++++++++----
drivers/md/bcache/super.c | 1 +
3 files changed, 10 insertions(+), 4 deletions(-)

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 122d975220945..bdf6071c1b184 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -841,15 +841,17 @@ 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;
}
@@ -884,10 +886,12 @@ 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 f7f8fb079d2a9..d73f9ea776861 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1511,6 +1511,7 @@ 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);
--
2.25.1