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

[Jens Axboe]

On 2014-05-16 09:15, Ming Lei wrote:
> On Fri, May 16, 2014 at 10:57 PM, Jens Axboe <axboe@xxxxxxxxx> wrote:
> > On 2014-05-16 08:53, Jens Axboe wrote:
> > > On 2014-05-15 06:33, 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.
> > >
> > >
> > > Why not just use blk_mq_start_hw_queues()?
> >
> >
> > Or, if you want to maintain current heuristics, just move the start and stop
> > under the vq_lock. That should prevent the race, as far as I can tell. Not
> > sure what that extra queue_stopped would buy you, seems a lot cleaner to
> > just maintain this state exclusively in the queue.
>
> Yes.
>
> But the flag can avoid to call blk_mq_start_stopped_hw_queues()
> unnecessarily, which needn't at most of times.  Considered that
> the interrupt may happen with very high frequency, I suggest to
> introduce the extra flag.

virtio-blk just has one queue, so the flag is at least pointless for 
now. And since the other code stops all of them anyway, I don't see any 
reason not to just rely on that.

--
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/