netdev - Re: [PATCH rcu/dev 3/3] net: Use call_rcu_flush() for dst_destroy

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Date:   Wed, 16 Nov 2022 19:44:41 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, Cong Wang <xiyou.wangcong@...il.com>,
        David Ahern <dsahern@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org,
        Paolo Abeni <pabeni@...hat.com>, rcu@...r.kernel.org,
        rostedt@...dmis.org, paulmck@...nel.org, fweisbec@...il.com
Subject: Re: [PATCH rcu/dev 3/3] net: Use call_rcu_flush() for dst_destroy_rcu

On Wed, Nov 16, 2022 at 7:16 PM Joel Fernandes (Google)
<joel@...lfernandes.org> wrote:
>
> In a networking test on ChromeOS, we find that using the new CONFIG_RCU_LAZY
> causes a networking test to fail in the teardown phase.
>
> The failure happens during: ip netns del <name>

And ? What happens then next ?

>
> Using ftrace, I found the callbacks it was queuing which this series fixes. Use
> call_rcu_flush() to revert to the old behavior. With that, the test passes.

What is this test about ? What barrier was used to make it not flaky ?

Was it depending on some undocumented RCU behavior ?

Maybe adding a sysctl to force the flush would be better for functional tests ?

I would rather change the test(s), than adding call_rcu_flush(),
adding merge conflicts to future backports.