Re: [Xen-devel] [PATCH] xen-blkfront: fix mq start/stop race

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Jun 22, 2017 at 09:36:52AM +0800, Junxiao Bi wrote:
> When ring buf full, hw queue will be stopped. While blkif interrupt consume
> request and make free space in ring buf, hw queue will be started again.
> But since start queue is protected by spin lock while stop not, that will
> cause a race.
>
> interrupt:                                      process:
> blkif_interrupt()                               blkif_queue_rq()
>  kick_pending_request_queues_locked()
>   blk_mq_start_stopped_hw_queues()
>    clear_bit(BLK_MQ_S_STOPPED, &hctx->state)
>                                                  blk_mq_stop_hw_queue(hctx)
>    blk_mq_run_hw_queue(hctx, async)
> 
> If ring buf is made empty in this case, interrupt will never come, then the
> hw queue will be stopped forever, all processes waiting for the pending io
> in the queue will hung.
> 
> Signed-off-by: Junxiao Bi <junxiao.bi@xxxxxxxxxx>
> Reviewed-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>

Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks, Roger.

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