Re: [PATCH] virtio-net: disable delayed refill when pausing rx

[Bui Quang Minh]

On 4/9/25 13:44, Bui Quang Minh wrote:
> On 4/8/25 14:34, Jason Wang wrote:
> > On Mon, Apr 7, 2025 at 10:27 AM Bui Quang Minh 
> > <minhquangbui99@xxxxxxxxx> wrote:
> > > On 4/7/25 08:03, Xuan Zhuo wrote:
> > > > On Fri,  4 Apr 2025 16:39:03 +0700, Bui Quang Minh 
> > > > <minhquangbui99@xxxxxxxxx> wrote:
> > > > > When pausing rx (e.g. set up xdp, xsk pool, rx resize), we call
> > > > > napi_disable() on the receive queue's napi. In delayed 
> > > > > refill_work, it
> > > > > also calls napi_disable() on the receive queue's napi. This can 
> > > > > leads to
> > > > > deadlock when napi_disable() is called on an already disabled 
> > > > > napi. This
> > > > > scenario can be reproducible by binding a XDP socket to virtio-net
> > > > > interface without setting up the fill ring. As a result, 
> > > > > try_fill_recv
> > > > > will fail until the fill ring is set up and refill_work is scheduled.
> > > > So, what is the problem? The refill_work is waiting? As I know, 
> > > > that thread
> > > > will sleep some time, so the cpu can do other work.
> > > When napi_disable is called on an already disabled napi, it will sleep
> > > in napi_disable_locked while still holding the netdev_lock. As a 
> > > result,
> > > later napi_enable gets stuck too as it cannot acquire the netdev_lock.
> > > This leads to refill_work and the pause-then-resume tx are stuck 
> > > altogether.
> > This needs to be added to the chagelog. And it looks like this is a 
> > fix for
> >
> > commit 413f0271f3966e0c73d4937963f19335af19e628
> > Author: Jakub Kicinski <kuba@xxxxxxxxxx>
> > Date:   Tue Jan 14 19:53:14 2025 -0800
> >
> >      net: protect NAPI enablement with netdev_lock()
> >
> > ?
>
> I'm not aware of this, will update the fix tags in the next patch.
>
> > I wonder if it's simpler to just hold the netdev lock in resize or xsk
> > binding instead of this.
>
> That looks cleaner, let me try that approach.

We need to hold the netdev_lock in the refill work too. Otherwise, we 
get this race

refill_work                                xdp bind
napi_disable()

                                            netdev_lock(dev)
                                            napi_disable_locked() <- hang
napi_enable() <- cannot acquire the lock


I don't like the idea to hold netdev_lock in refill_work. I will reply 
with the patch for you to review.

Thanks,
Quang Minh.