From 70904263512a74a3b8941dd9e6e515ca6fc57821 Mon Sep 17 00:00:00 2001 From: Ross Lagerwall Date: Fri, 14 Jul 2023 11:11:06 +0100 Subject: blk-mq: Fix stall due to recursive flush plug We have seen rare IO stalls as follows: * blk_mq_plug_issue_direct() is entered with an mq_list containing two requests. * For the first request, it sets last == false and enters the driver's queue_rq callback. * The driver queue_rq callback indirectly calls schedule() which calls blk_flush_plug(). This may happen if the driver has the BLK_MQ_F_BLOCKING flag set and is allowed to sleep in ->queue_rq. * blk_flush_plug() handles the remaining request in the mq_list. mq_list is now empty. * The original call to queue_rq resumes (with last == false). * The loop in blk_mq_plug_issue_direct() terminates because there are no remaining requests in mq_list. The IO is now stalled because the last request submitted to the driver had last == false and there was no subsequent call to commit_rqs(). Fix this by returning early in blk_mq_flush_plug_list() if rq_count is 0 which it will be in the recursive case, rather than checking if the mq_list is empty. At the same time, adjust one of the callers to skip the mq_list empty check as it is not necessary. Fixes: dc5fc361d891 ("block: attempt direct issue of plug list") Signed-off-by: Ross Lagerwall Reviewed-by: Bart Van Assche Link: https://lore.kernel.org/r/20230714101106.3635611-1-ross.lagerwall@citrix.com Signed-off-by: Jens Axboe --- block/blk-core.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) (limited to 'block/blk-core.c') diff --git a/block/blk-core.c b/block/blk-core.c index 99d8b9812b18..90de50082146 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -1144,8 +1144,7 @@ void __blk_flush_plug(struct blk_plug *plug, bool from_schedule) { if (!list_empty(&plug->cb_list)) flush_plug_callbacks(plug, from_schedule); - if (!rq_list_empty(plug->mq_list)) - blk_mq_flush_plug_list(plug, from_schedule); + blk_mq_flush_plug_list(plug, from_schedule); /* * Unconditionally flush out cached requests, even if the unplug * event came from schedule. Since we know hold references to the -- cgit v1.2.3