netdev - Re: [PATCH net v2 1/2] net/sched: act_mirred: use the backlog for mirred ingress

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Message-ID: <Zc4NgKlF_wT5578J@nanopsycho>
Date: Thu, 15 Feb 2024 14:11:28 +0100
From: Jiri Pirko <jiri@...nulli.us>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Jakub Kicinski <kuba@...nel.org>, davem@...emloft.net,
	netdev@...r.kernel.org, edumazet@...gle.com,
	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
	Davide Caratti <dcaratti@...hat.com>, jhs@...atatu.com,
	xiyou.wangcong@...il.com, shmulik.ladkani@...il.com
Subject: Re: [PATCH net v2 1/2] net/sched: act_mirred: use the backlog for
 mirred ingress

Thu, Feb 15, 2024 at 01:56:12PM CET, pabeni@...hat.com wrote:
>On Wed, 2024-02-14 at 07:04 -0800, Jakub Kicinski wrote:
>> On Wed, 14 Feb 2024 09:51:00 +0100 Jiri Pirko wrote:
>> > Wed, Feb 14, 2024 at 04:38:47AM CET, kuba@...nel.org wrote:
>> > > The test Davide added in commit ca22da2fbd69 ("act_mirred: use the backlog
>> > > for nested calls to mirred ingress") hangs our testing VMs every 10 or so
>> > > runs, with the familiar tcp_v4_rcv -> tcp_v4_rcv deadlock reported by
>> > > lockdep.
>> > > 
>> > > In the past there was a concern that the backlog indirection will
>> > > lead to loss of error reporting / less accurate stats. But the current
>> > > workaround does not seem to address the issue.  
>> > 
>> > Okay, so what the patch actually should change to fix this?
>> 
>> Sorry I'm not sure what you're asking.
>> 
>> We can't redirect traffic back to ourselves because we can end up
>> trying to take the socket lock for a socket that is generating
>> the packet.
>> 
>> Or are you asking how we can get the stats from the packet
>> asynchronously? We could build a local async scheme but I'd rather
>> not go there unless someone actually cares about these stats.
>
>I *guess* Jiri is suggesting to expand the commit message describing
>how the fix implemented by this patch works.
>
>@Jiri, feel free to provide the actual correct interpretation :)

Yes, but I was silent not to get beaten by another maintainer for
pointing this out :) But really, the patch desctiption should make it
simpler to understand the code, that's my motivation. Not to bug
people...


>
>Cheers,
>
>Paolo
>