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 16:35:52 +0800
From: Jason Wang <jasowang@...hat.com>
To: Heng Qi <hengqi@...ux.alibaba.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

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?

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

Thanks