SCHED_RESTART code path is relied to re-run queue for dispatch requests in hctx->dispatch. Meantime the SCHED_RSTART flag is checked when adding requests to hctx->dispatch. memory barriers have to be used for ordering the following two pair of OPs: 1) adding requests to hctx->dispatch and checking SCHED_RESTART in blk_mq_dispatch_rq_list() 2) clearing SCHED_RESTART and checking if there is request in hctx->dispatch in blk_mq_sched_restart(). Without the added memory barrier, either: 1) blk_mq_sched_restart() may miss requests added to hctx->dispatch meantime blk_mq_dispatch_rq_list() observes SCHED_RESTART, and not run queue in dispatch side or 2) blk_mq_dispatch_rq_list still sees SCHED_RESTART, and not run queue in dispatch side, meantime checking if there is request in hctx->dispatch from blk_mq_sched_restart() is missed. IO hang in ltp/fs_fill test is reported by kernel test robot: https://lkml.org/lkml/2020/7/26/77 Turns out it is caused by the above out-of-order OPs. And the IO hang can't be observed any more after applying this patch. Cc: Bart Van Assche <bvanassche@xxxxxxx> Cc: Christoph Hellwig <hch@xxxxxx> Cc: David Jeffery <djeffery@xxxxxxxxxx> Reported-by: kernel test robot <rong.a.chen@xxxxxxxxx> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx> --- block/blk-mq-sched.c | 9 +++++++++ block/blk-mq.c | 9 +++++++++ 2 files changed, 18 insertions(+) diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c index a19cdf159b75..d2790e5b06d1 100644 --- a/block/blk-mq-sched.c +++ b/block/blk-mq-sched.c @@ -78,6 +78,15 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx) return; clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state); + /* + * Order clearing SCHED_RESTART and list_empty_careful(&hctx->dispatch) + * in blk_mq_run_hw_queue(). Its pair is the barrier in + * blk_mq_dispatch_rq_list(). So dispatch code won't see SCHED_RESTART, + * meantime new request added to hctx->dispatch is missed to check in + * blk_mq_run_hw_queue(). + */ + smp_mb(); + blk_mq_run_hw_queue(hctx, true); } diff --git a/block/blk-mq.c b/block/blk-mq.c index 0015a1892153..6c1c3ad175a9 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -1437,6 +1437,15 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list, list_splice_tail_init(list, &hctx->dispatch); spin_unlock(&hctx->lock); + /* + * Order adding requests to hctx->dispatch and checking + * SCHED_RESTART flag. The pair of this smp_mb() is the one + * in blk_mq_sched_restart(). Avoid restart code path to + * miss the new added requests to hctx->dispatch, meantime + * SCHED_RESTART is observed here. + */ + smp_mb(); + /* * If SCHED_RESTART was set by the caller of this function and * it is no longer set that means that it was cleared by another -- 2.25.2
