When __sbq_wake_up() decrements wait_cnt to 0 but races with someone
else waking the waiter on the waitqueue (so the waitqueue becomes
empty), it exits without reseting wait_cnt to wake_batch number. Once
wait_cnt is 0, nobody will ever reset the wait_cnt or wake the new
waiters resulting in possible deadlocks or busyloops. Fix the problem by
making sure we reset wait_cnt even if we didn't wake up anybody in the
end.
Fixes: 040b83fcecfb ("sbitmap: fix possible io hung due to lost wakeup")
Reported-by: Keith Busch <kbusch@xxxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
lib/sbitmap.c | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index a39b1a877366..47cd8fb894ba 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -604,6 +604,7 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
struct sbq_wait_state *ws;
unsigned int wake_batch;
int wait_cnt;
+ bool ret;
ws = sbq_wake_ptr(sbq);
if (!ws)
@@ -614,12 +615,23 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
* For concurrent callers of this, callers should call this function
* again to wakeup a new batch on a different 'ws'.
*/
- if (wait_cnt < 0 || !waitqueue_active(&ws->wait))
+ if (wait_cnt < 0)
return true;
+ /*
+ * If we decremented queue without waiters, retry to avoid lost
+ * wakeups.
+ */
if (wait_cnt > 0)
- return false;
+ return !waitqueue_active(&ws->wait);
+ /*
+ * When wait_cnt == 0, we have to be particularly careful as we are
+ * responsible to reset wait_cnt regardless whether we've actually
+ * woken up anybody. But in case we didn't wakeup anybody, we still
+ * need to retry.
+ */
+ ret = !waitqueue_active(&ws->wait);
wake_batch = READ_ONCE(sbq->wake_batch);
/*
@@ -648,7 +660,7 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
sbq_index_atomic_inc(&sbq->wake_index);
atomic_set(&ws->wait_cnt, wake_batch);
- return false;
+ return ret;
}
void sbitmap_queue_wake_up(struct sbitmap_queue *sbq)
--
2.35.3
