netdev - Re: [PATCH net] virtio-net: set queues after driver

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Date: Wed, 9 Aug 2023 14:45:17 +0800
From: Jason Wang <jasowang@...hat.com>
To: Dragos Tatulea <dtatulea@...dia.com>
Cc: "mst@...hat.com" <mst@...hat.com>, 
	"xuanzhuo@...ux.alibaba.com" <xuanzhuo@...ux.alibaba.com>, 
	"virtualization@...ts.linux-foundation.org" <virtualization@...ts.linux-foundation.org>, 
	"edumazet@...gle.com" <edumazet@...gle.com>, "davem@...emloft.net" <davem@...emloft.net>, 
	"kuba@...nel.org" <kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>, 
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net] virtio-net: set queues after driver_ok

On Wed, Aug 9, 2023 at 2:23 PM Dragos Tatulea <dtatulea@...dia.com> wrote:
>
> On Tue, 2023-08-08 at 23:13 -0400, Jason Wang wrote:
> > Commit 25266128fe16 ("virtio-net: fix race between set queues and
> > probe") tries to fix the race between set queues and probe by calling
> > _virtnet_set_queues() before DRIVER_OK is set. This violates virtio
> > spec. Fixing this by setting queues after virtio_device_ready().
> >
> > Fixes: 25266128fe16 ("virtio-net: fix race between set queues and probe")
> > Reported-by: Dragos Tatulea <dtatulea@...dia.com>
> > Signed-off-by: Jason Wang <jasowang@...hat.com>
> > ---
> > The patch is needed for -stable.
> > ---
> >  drivers/net/virtio_net.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 1270c8d23463..ff03921e46df 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -4219,8 +4219,6 @@ static int virtnet_probe(struct virtio_device *vdev)
> >         if (vi->has_rss || vi->has_rss_hash_report)
> >                 virtnet_init_default_rss(vi);
> >
> > -       _virtnet_set_queues(vi, vi->curr_queue_pairs);
> > -
> >         /* serialize netdev register + virtio_device_ready() with ndo_open()
> > */
> >         rtnl_lock();
> >
> > @@ -4233,6 +4231,8 @@ static int virtnet_probe(struct virtio_device *vdev)
> >
> >         virtio_device_ready(vdev);
> >
> > +       _virtnet_set_queues(vi, vi->curr_queue_pairs);
> > +
> >         /* a random MAC address has been assigned, notify the device.
> >          * We don't fail probe if VIRTIO_NET_F_CTRL_MAC_ADDR is not there
> >          * because many devices work fine without getting MAC explicitly
>
> Thanks for the quick fix. Doesn't this fix though bring back the original race
> that was fixed in commit 25266128fe16 ("virtio-net: fix race between set queues
> and probe")? Or is being under the same rntl_lock session as register_netdev
> enough to avoid the race?

Yes, rtnl needs to be held for userspace requests to change the number
of queues. So we are serialized in this way.

Thanks

>
> Thanks,
> Dragos