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

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Date: Wed, 6 Dec 2023 17:11:02 +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 7:06 PM Heng Qi <hengqi@...ux.alibaba.com> wrote:
>
>
>
> 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).

Can we use rtnl_trylock() and reschedule the work?

>
> >> 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.

It is a "bug" that we need to fix.

>
> > 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?

So far no, you can refer to the past discussions, it might require the
collaboration from the uAPI and stack.

Thanks

>
> Thanks!
>
> >
> > Thanks
>