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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 06/23/2017 12:58 AM, Junxiao Bi wrote:
> Hi Boris & Juergen,
>
> Could you help review this patch? This is a race and will cause the io hung.
>
> Thanks,
> Junxiao.
>
> On 06/22/2017 09:36 AM, 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>

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

but I think Roger (copied) needs to Ack it.

-boris


> > ---
> >   drivers/block/xen-blkfront.c |    2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> > index 8bb160cd00e1..4767b82b2cf6 100644
> > --- a/drivers/block/xen-blkfront.c
> > +++ b/drivers/block/xen-blkfront.c
> > @@ -912,8 +912,8 @@ out_err:
> >         return BLK_MQ_RQ_QUEUE_ERROR;
> >   
> >   out_busy:
> > -       spin_unlock_irqrestore(&rinfo->ring_lock, flags);
> >         blk_mq_stop_hw_queue(hctx);
> > +       spin_unlock_irqrestore(&rinfo->ring_lock, flags);
> >         return BLK_MQ_RQ_QUEUE_BUSY;
> >   }
> >   
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