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

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Message-ID: <30419bd6-13b1-4426-9f93-b38b66ef7c3a@gmail.com>
Date: Mon, 7 Apr 2025 09:27:41 +0700
From: Bui Quang Minh <minhquangbui99@...il.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: "Michael S . Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
 Andrew Lunn <andrew+netdev@...n.ch>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.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 4/7/25 08:03, Xuan Zhuo wrote:
> On Fri,  4 Apr 2025 16:39:03 +0700, Bui Quang Minh <minhquangbui99@...il.com> 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.

Thanks,
Quang Minh.