Re: [PATCH 1/2] blk-mq: make sure hctx->next_cpu is set correctly

Jens Axboe
2018-01-17T15:45:44Z

On 1/17/18 5:34 AM, Ming Lei wrote:
> 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 is triggered 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.
>
> This patch deals with this issue by re-selecting next CPU, and make
> 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: 20e4d81393 ("blk-mq: simplify queue mapping & schedule with each possisble CPU")
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
> block/blk-mq.c | 31 +++++++++++++++++++++++++++++--
> 1 file changed, 29 insertions(+), 2 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index c376d1b6309a..dc4066d28323 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1416,21 +1416,48 @@ 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);
>
> - hctx->next_cpu = next_cpu;
> + /*
> + * No online CPU is found here, and this may happen when
> + * running from blk_mq_hctx_notify_dead(), and 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;

Since this should _only_ happen from the offline notification, why don't
we move the reset/update logic in that path and out of the hot path?

--
Jens Axboe