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

[Jason Wang]

On 2019/4/23 上午4:14, Dan Streetman wrote:
> On Sun, Apr 21, 2019 at 10:50 PM Jason Wang <address@hidden> wrote:
> > On 2019/4/17 上午2:46, 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.
> >
> > I think we need figure out why there are multiple vhost_net_stop() calls
> > simultaneously. E.g vhost-user register fd handlers like:
> >
> >           qemu_chr_fe_set_handlers(&s->chr, NULL, NULL,
> >                                    net_vhost_user_event, NULL, nc0->name,
> > NULL,
> >                                    true);
> >
> > which uses default main context, so it should only be called only in
> > main thread.
> net_vhost_user_event() schedules chr_closed_bh() to do its bottom half
> work; does aio_bh_schedule_oneshot() execute its events from the main
> thread?


I think so if net_vhost_user_event() was called in main thread (it calls 
qemu_get_current_aio_context()).


> For reference, the call chain is:
>
> chr_closed_bh()
>    qmp_set_link()
>      nc->info->link_status_changed() -> virtio_net_set_link_status()
>        virtio_net_set_status()
>          virtio_net_vhost_status()


The code was added by Marc since:

commit e7c83a885f865128ae3cf1946f8cb538b63cbfba
Author: Marc-André Lureau <address@hidden>
Date:   Mon Feb 27 14:49:56 2017 +0400

    vhost-user: delay vhost_user_stop

Cc him for more thoughts.

Thanks


> > Thanks
> >
> >
> > >    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.
> > >
> > > 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;