netdev - Re: [PATCH net-next v4 3/6] virtio_net: Add a lock for the command VQ.

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Message-ID: <72f6c8a55adac52ad17dfe11a579b5b3d5dc3cec.camel@redhat.com>
Date: Thu, 18 Apr 2024 17:48:57 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Dan Jurgens <danielj@...dia.com>, Heng Qi <hengqi@...ux.alibaba.com>, 
	Jason Wang <jasowang@...hat.com>
Cc: "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "mst@...hat.com"
 <mst@...hat.com>, "xuanzhuo@...ux.alibaba.com"
 <xuanzhuo@...ux.alibaba.com>,  "virtualization@...ts.linux.dev"
 <virtualization@...ts.linux.dev>, "davem@...emloft.net"
 <davem@...emloft.net>,  "edumazet@...gle.com" <edumazet@...gle.com>,
 "kuba@...nel.org" <kuba@...nel.org>, Jiri Pirko <jiri@...dia.com>
Subject: Re: [PATCH net-next v4 3/6] virtio_net: Add a lock for the command
 VQ.

On Thu, 2024-04-18 at 15:38 +0000, Dan Jurgens wrote:
> > From: Paolo Abeni <pabeni@...hat.com>
> > Sent: Thursday, April 18, 2024 5:57 AM
> > On Thu, 2024-04-18 at 15:36 +0800, Heng Qi wrote:
> > > 
> > > 在 2024/4/18 下午2:42, Jason Wang 写道:
> > > > On Wed, Apr 17, 2024 at 3:31 AM Daniel Jurgens <danielj@...dia.com>
> > wrote:
> > > > > The command VQ will no longer be protected by the RTNL lock. Use a
> > > > > spinlock to protect the control buffer header and the VQ.
> > > > > 
> > > > > Signed-off-by: Daniel Jurgens <danielj@...dia.com>
> > > > > Reviewed-by: Jiri Pirko <jiri@...dia.com>
> > > > > ---
> > > > >   drivers/net/virtio_net.c | 6 +++++-
> > > > >   1 file changed, 5 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > > > index 0ee192b45e1e..d02f83a919a7 100644
> > > > > --- a/drivers/net/virtio_net.c
> > > > > +++ b/drivers/net/virtio_net.c
> > > > > @@ -282,6 +282,7 @@ struct virtnet_info {
> > > > > 
> > > > >          /* Has control virtqueue */
> > > > >          bool has_cvq;
> > > > > +       spinlock_t cvq_lock;
> > > > Spinlock is instead of mutex which is problematic as there's no
> > > > guarantee on when the driver will get a reply. And it became even
> > > > more serious after 0d197a147164 ("virtio-net: add cond_resched() to
> > > > the command waiting loop").
> > > > 
> > > > Any reason we can't use mutex?
> > > 
> > > Hi Jason,
> > > 
> > > I made a patch set to enable ctrlq's irq on top of this patch set,
> > > which removes cond_resched().
> > > 
> > > But I need a little time to test, this is close to fast. So could the
> > > topic about cond_resched + spin lock or mutex lock be wait?
> > 
> > The big problem is that until the cond_resched() is there, replacing the
> > mutex with a spinlock can/will lead to scheduling while atomic splats. We
> > can't intentionally introduce such scenario.
> 
> When I created the series set_rx_mode wasn't moved to a work queue, 
> and the cond_resched wasn't there. 

Unfortunately cond_resched() is there right now.

> Mutex wasn't possible, then. If the CVQ is made to be event driven, then 
> the lock can be released right after posting the work to the VQ.

That should work.

> > Side note: the compiler apparently does not like guard() construct, leading to
> > new warning, here and in later patches. I'm unsure if the code simplification
> > is worthy.
> 
> I didn't see any warnings with GCC or clang. This is used other places in the kernel as well.
> gcc version 13.2.1 20230918 (Red Hat 13.2.1-3) (GCC)
> clang version 17.0.6 (Fedora 17.0.6-2.fc39)
> 

See:

https://patchwork.kernel.org/project/netdevbpf/patch/20240416193039.272997-4-danielj@nvidia.com/
https://netdev.bots.linux.dev/static/nipa/845178/13632442/build_32bit/stderr
https://netdev.bots.linux.dev/static/nipa/845178/13632442/build_allmodconfig_warn/stderr

Cheers,

Paolo