netdev - Re: [PATCH rcu/dev 1/3] net: Use call_rcu_flush() for qdisc_free

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Message-ID: <CANn89iJuy=PuAiwrjF3qZY0M+86eRQ=o_x-m-eoxOdyAM8yoSg@mail.gmail.com>
Date:   Thu, 17 Nov 2022 13:44:12 -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 1/3] net: Use call_rcu_flush() for qdisc_free_cb

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>
>
> 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.
>
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> ---
>  net/sched/sch_generic.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index a9aadc4e6858..63fbf640d3b2 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -1067,7 +1067,7 @@ static void qdisc_destroy(struct Qdisc *qdisc)
>
>         trace_qdisc_destroy(qdisc);
>
> -       call_rcu(&qdisc->rcu, qdisc_free_cb);
> +       call_rcu_flush(&qdisc->rcu, qdisc_free_cb);
>  }

I took a look at this one.

qdisc_free_cb() is essentially freeing : Some per-cpu memory, and the
'struct Qdisc'

I do not see why we need to force a flush for this (small ?) piece of memory.