The patch below does not apply to the 4.14-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From d7d8535f377e9ba87edbf7fbbd634ac942f3f54f Mon Sep 17 00:00:00 2001

From: Ming Lei ming.lei@redhat.com Date: Mon, 17 Aug 2020 18:01:15 +0800 Subject: [PATCH] blk-mq: order adding requests to hctx->dispatch and checking SCHED_RESTART

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.

Fixes: bd166ef183c2 ("blk-mq-sched: add framework for MQ capable IO schedulers") Reported-by: kernel test robot rong.a.chen@intel.com Signed-off-by: Ming Lei ming.lei@redhat.com Reviewed-by: Christoph Hellwig hch@lst.de Cc: Bart Van Assche bvanassche@acm.org Cc: Christoph Hellwig hch@lst.de Cc: David Jeffery djeffery@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Jens Axboe axboe@kernel.dk

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 35f8d0692442..a80f4986e594 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