Re: [Bug] double ->queue_rq() because of timeout in ->queue_rq()

Ming Lei
2022-10-21T02:25:21Z

On Thu, Oct 20, 2022 at 04:01:11PM -0400, Stefan Hajnoczi wrote:
> On Thu, Oct 20, 2022 at 05:10:13PM +0800, Ming Lei wrote:
> > Hi,
> >
> > David Jeffery found one double ->queue_rq() issue, so far it can
> > be triggered in the following two cases:
> >
> > 1) scsi driver in guest kernel
> >
> > - the story could be long vmexit latency or long preempt latency of
> > vCPU pthread, then IO req is timed out before queuing the request
> > to hardware but after calling blk_mq_start_request() during ->queue_rq(),
> > then timeout handler handles it by requeue, then double ->queue_rq() is
> > caused, and kernel panic
> >
> > 2) burst of kernel messages from irq handler
> >
> > For 1), I think it is one reasonable case, given latency from host side
> > can come anytime in theory because vCPU is emulated by one normal host
> > pthread which can be preempted anywhere. For 2), I guess kernel message is
> > supposed to be rate limited.
> >
> > Firstly, is this kind of so long(30sec) random latency when running kernel
> > code something normal? Or do we need to take care of it? IMO, it looks
> > reasonable in case of VM, but our VM experts may have better idea about this
> > situation. Also the default 30sec timeout could be reduced via sysfs or
> > drivers.
>
> 30 seconds is a long latency that does not occur during normal
> operation, but unfortunately does happen on occasion.

Thanks for the confirmation!

>
> I think there's an interest in understanding the root cause and solving
> long latencies (if possible) in the QEMU/KVM communities. We can
> investigate specific cases on kvm@vger.kernel.org and/or
> qemu-devel@nongnu.org.

The issue was original reported on VMware VM, but maybe David can figure
out how to trigger it on QEMU/KVM.

Thanks,
Ming