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rq-qos: fix missed wake-ups in rq_qos_throttle

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We saw a hang in production with WBT where there was only one waiter in
the throttle path and no outstanding IO.  This is because of the
has_sleepers optimization that is used to make sure we don't steal an
inflight counter for new submitters when there are people already on the
list.

We can race with our check to see if the waitqueue has any waiters (this
is done locklessly) and the time we actually add ourselves to the
waitqueue.  If this happens we'll go to sleep and never be woken up
because nobody is doing IO to wake us up.

Fix this by checking if the waitqueue has a single sleeper on the list
after we add ourselves, that way we have an uptodate view of the list.

Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Josef Bacik 2019-07-16 16:19:26 -04:00 committed by Jens Axboe
parent a6d81d30d3
commit 545fbd0775
1 changed files with 1 additions and 0 deletions
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1
block/blk-rq-qos.c
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@ -244,6 +244,7 @@ void rq_qos_wait(struct rq_wait *rqw, void *private_data,
return;
prepare_to_wait_exclusive(&rqw->wait, &data.wq, TASK_UNINTERRUPTIBLE);
has_sleeper = !wq_has_single_sleeper(&rqw->wait);
do {
if (data.got_token)
break;