From 48c033314f372478548203c583529f53080fd078 Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Thu, 8 Sep 2022 15:09:37 +0200 Subject: [PATCH] sbitmap: Avoid leaving waitqueue in invalid state in __sbq_wake_up() 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 Signed-off-by: Jan Kara Link: https://lore.kernel.org/r/20220908130937.2795-1-jack@suse.cz Signed-off-by: Jens Axboe --- lib/sbitmap.c | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-) diff --git a/lib/sbitmap.c b/lib/sbitmap.c index a39b1a87..47cd8fb 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) -- 2.7.4