Re: [Qemu-devel] [PATCH 1/2] add VirtIONet vhost_stopped flag to prevent multiple stops

[Dan Streetman]

On Fri, Apr 19, 2019 at 7:14 PM Michael S. Tsirkin <address@hidden> wrote:
>
> On Tue, Apr 16, 2019 at 02:46:23PM -0400, Dan Streetman wrote:
> > From: Dan Streetman <address@hidden>
> >
> > Buglink: https://launchpad.net/bugs/1823458
> >
> > There is a race condition when using the vhost-user driver, between a guest
> > shutdown and the vhost-user interface being closed.  This is explained in
> > more detail at the bug link above; the short explanation is the vhost-user
> > device can be closed while the main thread is in the middle of stopping
> > the vhost_net.  In this case, the main thread handling shutdown will
> > enter virtio_net_vhost_status() and move into the n->vhost_started (else)
> > block, and call vhost_net_stop(); while it is running that function,
> > another thread is notified that the vhost-user device has been closed,
> > and (indirectly) calls into virtio_net_vhost_status() also.  Since the
> > vhost_net status hasn't yet changed, the second thread also enters
> > the n->vhost_started block, and also calls vhost_net_stop().  This
> > causes problems for the second thread when it tries to stop the network
> > that's already been stopped.
> >
> > This adds a flag to the struct that's atomically set to prevent more than
> > one thread from calling vhost_net_stop().  The atomic_fetch_inc() is likely
> > overkill and probably could be done with a simple check-and-set, but
> > since it's a race condition there would still be a (very, very) small
> > window without using an atomic to set it.
>
> How? Isn't all this under the BQL?

I don't think so, although I'm not deeply familiar with the code.
Note the code path listed in my last email, run from
aio_bh_schedule_oneshot() - does that hold the bql while running?

>
> >
> > Signed-off-by: Dan Streetman <address@hidden>
> > ---
> >  hw/net/virtio-net.c            | 3 ++-
> >  include/hw/virtio/virtio-net.h | 1 +
> >  2 files changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > index ffe0872fff..d36f50d5dd 100644
> > --- a/hw/net/virtio-net.c
> > +++ b/hw/net/virtio-net.c
> > @@ -13,6 +13,7 @@
> >
> >  #include "qemu/osdep.h"
> >  #include "qemu/iov.h"
> > +#include "qemu/atomic.h"
> >  #include "hw/virtio/virtio.h"
> >  #include "net/net.h"
> >  #include "net/checksum.h"
> > @@ -240,7 +241,7 @@ static void virtio_net_vhost_status(VirtIONet *n, 
> > uint8_t status)
> >                           "falling back on userspace virtio", -r);
> >              n->vhost_started = 0;
> >          }
> > -    } else {
> > +    } else if (atomic_fetch_inc(&n->vhost_stopped) == 0) {
> >          vhost_net_stop(vdev, n->nic->ncs, queues);
> >          n->vhost_started = 0;
> >      }
> > diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
> > index b96f0c643f..d03fd933d0 100644
> > --- a/include/hw/virtio/virtio-net.h
> > +++ b/include/hw/virtio/virtio-net.h
> > @@ -164,6 +164,7 @@ struct VirtIONet {
> >      uint8_t nouni;
> >      uint8_t nobcast;
> >      uint8_t vhost_started;
> > +    int vhost_stopped;
> >      struct {
> >          uint32_t in_use;
> >          uint32_t first_multi;
>
> OK questions same as any state:
>
> - do we need to migrate this?
> - reset it on device reset?
>
> > --
> > 2.20.1