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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Stefan Haberland <sth@linux.vnet.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Christoph Hellwig <hch@lst.de>,
        Sagi Grimberg <sagi@grimberg.me>,
        Ming Lei <ming.lei@redhat.com>, Jens Axboe <axboe@kernel.dk>
Subject: [PATCH 4.16 42/68] blk-mq: make sure that correct hctx->next_cpu is set
Date:   Tue, 17 Apr 2018 17:57:55 +0200
Message-Id: <20180417155751.044663858@linuxfoundation.org>
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4.16-stable review patch.  If anyone has any objections, please let me know.

------------------

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

commit a1c735fb790745f94a359df45c11df4a69760389 upstream.

>From commit 20e4d81393196 (blk-mq: simplify queue mapping & schedule
with each possisble CPU), one hctx can be mapped from all offline CPUs,
then hctx->next_cpu can be set as wrong.

This patch fixes this issue by making hctx->next_cpu pointing to the
first CPU in hctx->cpumask if all CPUs in hctx->cpumask are offline.

Cc: Stefan Haberland <sth@linux.vnet.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Fixes: 20e4d81393196 ("blk-mq: simplify queue mapping & schedule with each possisble CPU")
Cc: stable@vger.kernel.org
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 block/blk-mq.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2439,6 +2439,8 @@ static void blk_mq_map_swqueue(struct re
 		 */
 		hctx->next_cpu = cpumask_first_and(hctx->cpumask,
 				cpu_online_mask);
+		if (hctx->next_cpu >= nr_cpu_ids)
+			hctx->next_cpu = cpumask_first(hctx->cpumask);
 		hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
 	}
 }