netdev - Re: [PATCH net-next v6 4/5] virtio-net: add spin lock for ctrl cmd access

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Date: Tue, 5 Dec 2023 19:05:59 +0800
From: Heng Qi <hengqi@...ux.alibaba.com>
To: Jason Wang <jasowang@...hat.com>
Cc: netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
 mst@...hat.com, pabeni@...hat.com, kuba@...nel.org,
 yinjun.zhang@...igine.com, edumazet@...gle.com, davem@...emloft.net,
 hawk@...nel.org, john.fastabend@...il.com, ast@...nel.org, horms@...nel.org,
 xuanzhuo@...ux.alibaba.com
Subject: Re: [PATCH net-next v6 4/5] virtio-net: add spin lock for ctrl cmd
 access



在 2023/12/5 下午4:35, Jason Wang 写道:
> On Tue, Dec 5, 2023 at 4:02 PM Heng Qi <hengqi@...ux.alibaba.com> wrote:
>> Currently access to ctrl cmd is globally protected via rtnl_lock and works
>> fine. But if dim work's access to ctrl cmd also holds rtnl_lock, deadlock
>> may occur due to cancel_work_sync for dim work.
> Can you explain why?

For example, during the bus unbind operation, the following call stack 
occurs:
virtnet_remove -> unregister_netdev -> rtnl_lock[1] -> virtnet_close -> 
cancel_work_sync -> virtnet_rx_dim_work -> rtnl_lock[2] (deadlock occurs).

>> Therefore, treating
>> ctrl cmd as a separate protection object of the lock is the solution and
>> the basis for the next patch.
> Let's don't do that. Reasons are:
>
> 1) virtnet_send_command() may wait for cvq commands for an indefinite time

Yes, I took that into consideration. But ndo_set_rx_mode's need for an 
atomic
environment rules out the mutex lock.

> 2) hold locks may complicate the future hardening works around cvq

Agree, but I don't seem to have thought of a better way besides passing 
the lock.
Do you have any other better ideas or suggestions?

Thanks!

>
> Thanks