Commit 7bed4595 authored Jan 18, 2018 by Ming Lei Committed by Jens Axboe Jan 17, 2018

blk-mq: make sure hctx->next_cpu is set correctly



When hctx->next_cpu is set from possible online CPUs, there is one
race in which hctx->next_cpu may be set as >= nr_cpu_ids, and finally
break workqueue.

The race can be triggered in the following two sitations:

1) when one CPU is becoming DEAD, blk_mq_hctx_notify_dead() is called
to dispatch requests from the DEAD cpu context, but at that
time, this DEAD CPU has been cleared from 'cpu_online_mask', so all
CPUs in hctx->cpumask may become offline, and cause hctx->next_cpu set
a bad value.

2) blk_mq_delay_run_hw_queue() is called from CPU B, and found the queue
should be run on the other CPU A, then CPU A may become offline at the
same time and all CPUs in hctx->cpumask become offline.

This patch deals with this issue by re-selecting next CPU, and making
sure it is set correctly.

Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Stefan Haberland <sth@linux.vnet.ibm.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Reported-by: "jianchao.wang" <jianchao.w.wang@oracle.com>
Tested-by: "jianchao.wang" <jianchao.w.wang@oracle.com>
Fixes: 20e4d813 ("blk-mq: simplify queue mapping & schedule with each possisble CPU")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent 85cf955d

block/blk-mq.c

+28 −2

Original line number	Diff line number	Diff line
		@@ -1319,21 +1319,47 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
		*/
		static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
		{
		bool tried = false;

		if (hctx->queue->nr_hw_queues == 1)
		return WORK_CPU_UNBOUND;

		if (--hctx->next_cpu_batch <= 0) {
		int next_cpu;

		select_cpu:
		next_cpu = cpumask_next_and(hctx->next_cpu, hctx->cpumask,
		cpu_online_mask);
		if (next_cpu >= nr_cpu_ids)
		next_cpu = cpumask_first_and(hctx->cpumask,cpu_online_mask);

		/*
		* No online CPU is found, so have to make sure hctx->next_cpu
		* is set correctly for not breaking workqueue.
		*/
		if (next_cpu >= nr_cpu_ids)
		hctx->next_cpu = cpumask_first(hctx->cpumask);
		else
		hctx->next_cpu = next_cpu;
		hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
		}

		/*
		* Do unbound schedule if we can't find a online CPU for this hctx,
		* and it should only happen in the path of handling CPU DEAD.
		*/
		if (!cpu_online(hctx->next_cpu)) {
		if (!tried) {
		tried = true;
		goto select_cpu;
		}

		/*
		* Make sure to re-select CPU next time once after CPUs
		* in hctx->cpumask become online again.
		*/
		hctx->next_cpu_batch = 1;
		return WORK_CPU_UNBOUND;
		}
		return hctx->next_cpu;
		}

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