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

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Message-ID: <20250408073709.4e054636@kernel.org>
Date: Tue, 8 Apr 2025 07:37:09 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Jason Wang <jasowang@...hat.com>, Bui Quang Minh
 <minhquangbui99@...il.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
 "Michael S . Tsirkin" <mst@...hat.com>, Andrew Lunn
 <andrew+netdev@...n.ch>, Eric Dumazet <edumazet@...gle.com>, Alexei
 Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend
 <john.fastabend@...il.com>, Eugenio Pérez
 <eperezma@...hat.com>, "David S . Miller" <davem@...emloft.net>,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
 virtualization@...ts.linux.dev
Subject: Re: [PATCH] virtio-net: disable delayed refill when pausing rx

On Tue, 8 Apr 2025 11:28:54 +0200 Paolo Abeni wrote:
> >> 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@...nel.org>
> > Date:   Tue Jan 14 19:53:14 2025 -0800
> > 
> >     net: protect NAPI enablement with netdev_lock()
> > 
> > ?
> > 
> > I wonder if it's simpler to just hold the netdev lock in resize or xsk
> > binding instead of this.  
> 
> Setting:
> 
> 	dev->request_ops_lock = true;
> 
> in virtnet_probe() before calling register_netdevice() should achieve
> the above. Could you please have a try?

Can we do that or do we need a more tailored fix? request_ops_lock only
appeared in 6.15 and the bug AFAIU dates back to 6.14. We don't normally
worry about given the stream of fixes - request_ops_lock is a bit risky.
Jason's suggestion AFAIU is just to wrap the disable/enable pairs in
the lock. We can try request_ops_lock in -next ?

Bui Quang Minh, could you add a selftest for this problem?
See tools/testing/selftests/drivers/net/virtio_net/
You can re-use / extend the XSK helper from
tools/testing/selftests/drivers/net/xdp_helper.c ?
(move it to tools/testing/selftests/net/lib for easier access)