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 <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>

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index a39b1a877366a4fd39597c1e2d10f25108334485..47cd8fb894baee9b02660c1b0df40fca455c00c0 100644 (file)
--- 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)