[Bionic][PATCH 3/3] blk-mq: turn WARN_ON in __blk_mq_run_hw_queue into printk

[Jose Ricardo Ziviani]

From: Ming Lei <ming.lei at redhat.com>

BugLink: http://bugs.launchpad.net/bugs/1759722

We know this WARN_ON is harmless and in reality it may be trigged,
so convert it to printk() and dump_stack() to avoid to confusing
people.

Also add comment about two releated races here.

Cc: Christian Borntraeger <borntraeger at de.ibm.com>
Cc: Stefan Haberland <sth at linux.vnet.ibm.com>
Cc: Christoph Hellwig <hch at lst.de>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: "jianchao.wang" <jianchao.w.wang at oracle.com>
Signed-off-by: Ming Lei <ming.lei at redhat.com>
Signed-off-by: Jens Axboe <axboe at kernel.dk>
(cherry picked from commit 7df938fbc4ee641e70e05002ac67c24b19e86e74)
Signed-off-by: Jose Ricardo Ziviani <joserz at linux.vnet.ibm.com>
---
 block/blk-mq.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index cce2e66bc729..002eb9d750d7 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1208,9 +1208,27 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
 	/*
 	 * We should be running this queue from one of the CPUs that
 	 * are mapped to it.
+	 *
+	 * There are at least two related races now between setting
+	 * hctx->next_cpu from blk_mq_hctx_next_cpu() and running
+	 * __blk_mq_run_hw_queue():
+	 *
+	 * - hctx->next_cpu is found offline in blk_mq_hctx_next_cpu(),
+	 *   but later it becomes online, then this warning is harmless
+	 *   at all
+	 *
+	 * - hctx->next_cpu is found online in blk_mq_hctx_next_cpu(),
+	 *   but later it becomes offline, then the warning can't be
+	 *   triggered, and we depend on blk-mq timeout handler to
+	 *   handle dispatched requests to this hctx
 	 */
-	WARN_ON(!cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask) &&
-		cpu_online(hctx->next_cpu));
+	if (!cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask) &&
+		cpu_online(hctx->next_cpu)) {
+		printk(KERN_WARNING "run queue from wrong CPU %d, hctx %s\n",
+			raw_smp_processor_id(),
+			cpumask_empty(hctx->cpumask) ? "inactive": "active");
+		dump_stack();
+	}
 
 	/*
 	 * We can't run the queue inline with ints disabled. Ensure that
-- 
2.14.3