FAILED: patch "[PATCH] blk-mq: order adding requests to hctx->dispatch and checking" failed to apply to 4.14-stable tree

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 <[email protected]>.

thanks,

greg k-h

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

From d7d8535f377e9ba87edbf7fbbd634ac942f3f54f Mon Sep 17 00:00:00 2001
From: Ming Lei <[email protected]>
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.

index a19cdf159b75..d2790e5b06d1 100644
@@ -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);
 }
 

index 35f8d0692442..a80f4986e594 100644
@@ -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