Patch "blk-mq: Always schedule hctx->next_cpu" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

    blk-mq: Always schedule hctx->next_cpu

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
blk-mq-always-schedule-hctx-next_cpu.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Gabriel Krisman Bertazi <[email protected]>
Date: Wed, 28 Sep 2016 00:24:24 -0300
Subject: blk-mq: Always schedule hctx->next_cpu

From: Gabriel Krisman Bertazi <[email protected]>

commit c02ebfdddbafa9a6a0f52fbd715e6bfa229af9d3 upstream.

Commit 0e87e58bf60e ("blk-mq: improve warning for running a queue on the wrong CPU") attempts to avoid triggering the WARN_ON in __blk_mq_run_hw_queue when the expected CPU is dead. Problem is, in the last batch execution before round robin, blk_mq_hctx_next_cpu can schedule a dead CPU and also update next_cpu to the next alive CPU in the mask, which will trigger the WARN_ON despite the previous workaround.

The following patch fixes this scenario by always scheduling the value in hctx->next_cpu. This changes the moment when we round-robin the CPU running the hctx, but it really doesn't matter, since it still executes BLK_MQ_CPU_WORK_BATCH times in a row before switching to another CPU.

---
 block/blk-mq.c |    4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

@@ -895,7 +895,7 @@ static int blk_mq_hctx_next_cpu(struct b
 		return WORK_CPU_UNBOUND;
 
 	if (--hctx->next_cpu_batch <= 0) {
-		int cpu = hctx->next_cpu, next_cpu;
+		int next_cpu;
 
 		next_cpu = cpumask_next(hctx->next_cpu, hctx->cpumask);
 		if (next_cpu >= nr_cpu_ids)

@@ -903,8 +903,6 @@ static int blk_mq_hctx_next_cpu(struct b
 
 		hctx->next_cpu = next_cpu;
 		hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
-
-		return cpu;
 	}
 
 	return hctx->next_cpu;

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/blk-mq-always-schedule-hctx-next_cpu.patch