Re: [Qemu-devel] [PATCH 0/4] Tweaks around virtio-blk start/stop

[Cornelia Huck <cornelia.huck@de.ibm.com>]

(re-adding cc:s)

On Tue, 22 Mar 2016 15:18:05 +0800
Fam Zheng <famz@redhat.com> wrote:

> On Tue, 03/22 15:10, tu bo wrote:
> > Hi Fam:
> > 
> > On 03/21/2016 06:57 PM, Fam Zheng wrote:
> > >On Thu, 03/17 19:03, tu bo wrote:
> > >>
> > >>On 03/17/2016 08:39 AM, Fam Zheng wrote:
> > >>>On Wed, 03/16 14:45, Paolo Bonzini wrote:
> > >>>>
> > >>>>
> > >>>>On 16/03/2016 14:38, Christian Borntraeger wrote:
> > >>>>>>If you just remove the calls to virtio_queue_host_notifier_read, here
> > >>>>>>and in virtio_queue_aio_set_host_notifier_fd_handler, does it work
> > >>>>>>(keeping patches 2-4 in)?
> > >>>>>
> > >>>>>With these changes and patch 2-4 it does no longer locks up.
> > >>>>>I keep it running some hour to check if a crash happens.
> > >>>>>
> > >>>>>Tu Bo, your setup is currently better suited for reproducing. Can you also check?
> > >>>>
> > >>>>Great, I'll prepare a patch to virtio then sketching the solution that
> > >>>>Conny agreed with.
> > >>>>
> > >>>>While Fam and I agreed that patch 1 is not required, I'm not sure if the
> > >>>>mutex is necessary in the end.
> > >>>
> > >>>If we can fix this from the virtio_queue_host_notifier_read side, the mutex/BH
> > >>>are not necessary; but OTOH the mutex does catch such bugs, so maybe it's good
> > >>>to have it. I'm not sure about the BH.
> > >>>
> > >>>And on a hindsight I realize we don't want patches 2-3 too. Actually the
> > >>>begin/end pair won't work as expected because of the blk_set_aio_context.
> > >>>
> > >>>Let's hold on this series.
> > >>>
> > >>>>
> > >>>>So if Tu Bo can check without the virtio_queue_host_notifier_read calls,
> > >>>>and both with/without Fam's patches, it would be great.
> > >>>
> > >>>Tu Bo, only with/withoug patch 4, if you want to check. Sorry for the noise.
> > >>>
> > >>1. without the virtio_queue_host_notifier_read calls,  without patch 4
> > >>
> > >>crash happens very often,
> > >>
> > >>(gdb) bt
> > >>#0  bdrv_co_do_rw (opaque=0x0) at block/io.c:2172
> > >>#1  0x000002aa165da37e in coroutine_trampoline (i0=<optimized out>,
> > >>i1=1812051552) at util/coroutine-ucontext.c:79
> > >>#2  0x000003ff7dd5150a in __makecontext_ret () from /lib64/libc.so.6
> > >>
> > >>
> > >>2. without the virtio_queue_host_notifier_read calls, with patch 4
> > >>
> > >>crash happens very often,
> > >>
> > >>(gdb) bt
> > >>#0  bdrv_co_do_rw (opaque=0x0) at block/io.c:2172
> > >>#1  0x000002aa39dda43e in coroutine_trampoline (i0=<optimized out>,
> > >>i1=-1677715600) at util/coroutine-ucontext.c:79
> > >>#2  0x000003ffab6d150a in __makecontext_ret () from /lib64/libc.so.6
> > >>
> > >>
> > >
> > >Tu Bo,
> > >
> > >Could you help test this patch (on top of master, without patch 4)?
> > >
> > >diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > >index 08275a9..47f8043 100644
> > >--- a/hw/virtio/virtio.c
> > >+++ b/hw/virtio/virtio.c
> > >@@ -1098,7 +1098,14 @@ void virtio_queue_notify_vq(VirtQueue *vq)
> > >
> > >  void virtio_queue_notify(VirtIODevice *vdev, int n)
> > >  {
> > >-    virtio_queue_notify_vq(&vdev->vq[n]);
> > >+    VirtQueue *vq = &vdev->vq[n];
> > >+    EventNotifier *n;
> > >+    n = virtio_queue_get_host_notifier(vq);
> > >+    if (n) {
> > >+        event_notifier_set(n);
> > >+    } else {
> > >+        virtio_queue_notify_vq(vq);
> > >+    }
> > >  }
> > >
> > >  uint16_t virtio_queue_vector(VirtIODevice *vdev, int n)
> > >
> > >
> > 
> > I got a build error as below,
> > 
> > /BUILD/qemu-2.5.50/hw/virtio/virtio.c: In function 'virtio_queue_notify':
> > /BUILD/qemu-2.5.50/hw/virtio/virtio.c:1102:20: error: 'n' redeclared
> > as different kind of symbol
> >      EventNotifier *n;
> >                     ^
> > /BUILD/qemu-2.5.50/hw/virtio/virtio.c:1099:50: note: previous
> > definition of 'n' was here
> >  void virtio_queue_notify(VirtIODevice *vdev, int n)
> > 
> > 
> > Then I did some change for your patch as below,
> > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > index 08275a9..a10da39 100644
> > --- a/hw/virtio/virtio.c
> > +++ b/hw/virtio/virtio.c
> > @@ -1098,7 +1098,14 @@ void virtio_queue_notify_vq(VirtQueue *vq)
> > 
> >  void virtio_queue_notify(VirtIODevice *vdev, int n)
> >  {
> > -    virtio_queue_notify_vq(&vdev->vq[n]);
> > +    VirtQueue *vq = &vdev->vq[n];
> > +    EventNotifier *en;
> > +    en = virtio_queue_get_host_notifier(vq);
> > +    if (en) {
> > +        event_notifier_set(en);
> > +    } else {
> > +        virtio_queue_notify_vq(vq);
> > +    }
> >  }
> > 
> >  uint16_t virtio_queue_vector(VirtIODevice *vdev, int n)
> > 
> > With qemu master + modified patch above(without patch 4, without
> > Conny's patches), I did NOT get crash so far.  thanks
> 
> Yes, it was a mistake.  Thanks for the testing!

I'm wondering what we learn from this. Bypassing notify_vq() removes
the race, but that's not the solution here.

So far, we had the best results with my refactoring + the mutex/bh
change. Two problems:

- We don't really understand yet why my refactoring helps, but passing
the assign value is a good canditate (and it's agreed that this fixes a
bug, I think.)
- There's some problem with the bh, if I understood Stefan correctly.