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Subject[PATCH 4.14 034/166] bcache: fix a lost wake-up problem caused by mca_cannibalize_lock
Date
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 e4a3f692057b8..a3763d664a67a 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -548,6 +548,7 @@ 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 9fca837d0b41e..fba0fff8040d6 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -840,15 +840,17 @@ 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;
}
@@ -883,10 +885,12 @@ 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 7fcc1ba12bc01..6bf1559a1f0db 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);
--
2.25.1


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