[PATCH] vhost: fix read vs write lock mismatch

[Stephen Hemminger]

On Mon, 25 Nov 2024 17:31:26 +0100
Maxime Coquelin <maxime.coquelin at redhat.com> wrote:

> On 11/25/24 17:28, David Marchand wrote:
> > On Mon, Nov 25, 2024 at 5:20 PM Maxime Coquelin
> > <maxime.coquelin at redhat.com> wrote:  
> >> On 11/25/24 12:14, David Marchand wrote:  
> >>> On Mon, Nov 18, 2024 at 5:24 PM Stephen Hemminger
> >>> <stephen at networkplumber.org> wrote:  
> >>>>
> >>>> If lock is acquired for write, it must be released for write
> >>>> or a deadlock is likely.
> >>>>
> >>>> Bugzilla ID: 1582
> >>>> Fixes: 9fc93a1e2320 ("vhost: fix virtqueue access check in datapath")
> >>>> Cc: david.marchand at redhat.com
> >>>> Cc: stable at dpdk.org
> >>>>
> >>>> Signed-off-by: Stephen Hemminger <stephen at networkplumber.org>
> >>>> ---
> >>>>    lib/vhost/virtio_net.c | 2 +-
> >>>>    1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
> >>>> index 298a5dae74..d764d4bc6a 100644
> >>>> --- a/lib/vhost/virtio_net.c
> >>>> +++ b/lib/vhost/virtio_net.c
> >>>> @@ -2538,7 +2538,7 @@ virtio_dev_rx_async_submit(struct virtio_net *dev, struct vhost_virtqueue *vq,
> >>>>
> >>>>           if (unlikely(!vq->access_ok)) {
> >>>>                   vhost_user_iotlb_rd_unlock(vq);
> >>>> -               rte_rwlock_read_unlock(&vq->access_lock);
> >>>> +               rte_rwlock_write_unlock(&vq->access_lock);  
> >>>
> >>> A write lock is taken earlier, because virtio_dev_rx_async_submit_*
> >>> need it for access to vq->async (as opposed to the sync code that only
> >>> takes read lock).
> >>>
> >>> Here, no need to release/take again all locks.
> >>> A simpler fix is to directly call vring_translate(dev, vq).
> >>>
> >>>  
> >>
> >> Ok, so both solutions are correct.
> >>
> >> David's one is more optimized, but this is a corner case in the async
> >> datapath so not really critical.
> >>
> >> On the other hand, Stephen's solution keeps the same pattern as the
> >> other datapaths.  
> > 
> > Ok, it avoids having a special case, and I prefer limiting special cases too.
> > 
> > I'll send a RFC for new lock annotations (for catching such bug) for
> > the next release.  
> 
> Great, thanks for that!
> 
> >   
> >>
> >> I'd go with Stephen's solution, but would change the commit title to:
> >>
> >> vhost: fix deadlock in Rx async path
> >>
> >> With this change:
> >>
> >> Reviewed-by: Maxime Coquelin <maxime.coquelin at redhat.com>  
> > 
> > Yes, the title needed some work.
> > 
> >   
> 
> Ack, will change while applying.

Yes better title would be great.