From 545fbd0775bafcefc8f7bc844291bd13c44b7fdc Mon Sep 17 00:00:00 2001 From: Josef Bacik Date: Tue, 16 Jul 2019 16:19:26 -0400 Subject: rq-qos: fix missed wake-ups in rq_qos_throttle 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 Signed-off-by: Josef Bacik Signed-off-by: Jens Axboe --- block/blk-rq-qos.c | 1 + 1 file changed, 1 insertion(+) (limited to 'block') diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c index 659ccb8b693fa..67a0a4c07060b 100644 --- a/block/blk-rq-qos.c +++ b/block/blk-rq-qos.c @@ -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; -- cgit v1.2.3