From: Ross Lagerwall ross.lagerwall@citrix.com
[ Upstream commit 70904263512a74a3b8941dd9e6e515ca6fc57821 ]
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 ross.lagerwall@citrix.com Reviewed-by: Bart Van Assche bvanassche@acm.org Link: https://lore.kernel.org/r/20230714101106.3635611-1-ross.lagerwall@citrix.com Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Sasha Levin sashal@kernel.org --- block/blk-core.c | 3 +-- block/blk-mq.c | 9 ++++++++- 2 files changed, 9 insertions(+), 3 deletions(-)
diff --git a/block/blk-core.c b/block/blk-core.c index 24ee7785a5ad5..ebb7a1689b261 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -1140,8 +1140,7 @@ 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 diff --git a/block/blk-mq.c b/block/blk-mq.c index add013d5bbdab..100fb0c3114f8 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -2749,7 +2749,14 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule) { struct request *rq;
- if (rq_list_empty(plug->mq_list)) + /* + * We may have been called recursively midway through handling + * plug->mq_list via a schedule() in the driver's queue_rq() callback. + * To avoid mq_list changing under our feet, clear rq_count early and + * bail out specifically if rq_count is 0 rather than checking + * whether the mq_list is empty. + */ + if (plug->rq_count == 0) return; plug->rq_count = 0;