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sbitmap: Avoid leaving waitqueue in invalid state in __sbq_wake_up()

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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: 040b83fcec ("sbitmap: fix possible io hung due to lost wakeup")
Reported-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220908130937.2795-1-jack@suse.cz
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jan Kara 2022-09-08 15:09:37 +02:00 committed by Jens Axboe
parent 8807707df7
commit 48c033314f
1 changed files with 15 additions and 3 deletions
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18
lib/sbitmap.c
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@ -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)