Re: [PATCH v1] virtio_blk: fix race between start and stop queue

[Jens Axboe]

On 2014-05-16 09:43, Ming Lei wrote:
> On Fri, May 16, 2014 at 11:32 PM, Jens Axboe <axboe@xxxxxxxxx> wrote:
> > On 2014-05-16 09:31, Ming Lei wrote:
> > > When there isn't enough vring descriptor for adding to vq,
> > > blk-mq will be put as stopped state until some of pending
> > > descriptors are completed & freed.
> > >
> > > Unfortunately, the vq's interrupt may come just before
> > > blk-mq's BLK_MQ_S_STOPPED flag is set, so the blk-mq will
> > > still be kept as stopped even though lots of descriptors
> > > are completed and freed in the interrupt handler. The worst
> > > case is that all pending descriptors are freed in the
> > > interrupt handler, and the queue is kept as stopped forever.
> > >
> > > This patch fixes the problem by starting/stopping blk-mq
> > > with holding vq_lock.
> >
> >
> > Thanks, this looks good, I'll apply it for 3.16 (with a stable marker, even
> > if it is an unlikely event).
>
> Thanks.
>
> It shouldn't be very difficult to happen in case of
> non-indirect descriptor, and it is easy to reproduce
> when module parameter of 'virtblk_queue_depth'
> is bigger than vq->num_free for non-indirect case.

I agree, it can definitely be setup so that it would not be hard to 
trigger. But I don't recall seeing any hang bugs since 3.13 was 
released, which would seem to indicate that it doesn't happen a lot in 
the wild with default settings.

--
Jens Axboe

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/