netdev - Re: [PATCH net] vrf: revert "vrf: Remove unnecessary RCU-bh critical section"

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Message-ID: <ZvkZ0Ex0k6_G6hNo@shredder.mtl.com>
Date: Sun, 29 Sep 2024 12:11:44 +0300
From: Ido Schimmel <idosch@...dia.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
	edumazet@...gle.com, pabeni@...hat.com, stable@...r.kernel.org,
	greearb@...delatech.com, fw@...len.de, dsahern@...nel.org,
	Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net] vrf: revert "vrf: Remove unnecessary RCU-bh critical
 section"

On Sun, Sep 29, 2024 at 02:18:20AM -0400, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@...gle.com>
> 
> This reverts commit 504fc6f4f7f681d2a03aa5f68aad549d90eab853.
> 
> dev_queue_xmit_nit is expected to be called with BH disabled.
> __dev_queue_xmit has the following:
> 
>         /* Disable soft irqs for various locks below. Also
>          * stops preemption for RCU.
>          */
>         rcu_read_lock_bh();
> 
> VRF must follow this invariant. The referenced commit removed this
> protection. Which triggered a lockdep warning:

[...]

> 
> Fixes: 504fc6f4f7f6 ("vrf: Remove unnecessary RCU-bh critical section")
> Link: https://lore.kernel.org/netdev/20240925185216.1990381-1-greearb@candelatech.com/
> Reported-by: Ben Greear <greearb@...delatech.com>
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> Cc: stable@...r.kernel.org

Reviewed-by: Ido Schimmel <idosch@...dia.com>
Tested-by: Ido Schimmel <idosch@...dia.com>

Thanks Willem!

The reason my script from 504fc6f4f7f6 did not trigger the problem is
that it was pinging the address inside the VRF, so vrf_finish_direct()
was only called from the Rx path.

If you ping the address outside of the VRF:

ping -I vrf1 -i 0.1 -c 10 -q 192.0.2.1

Then vrf_finish_direct() is called from process context and the lockdep
warning is triggered. Tested that it does not trigger after applying the
revert.