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

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Date:   Fri, 18 Nov 2022 00:23:59 +0000
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Eric Dumazet <edumazet@...gle.com>
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 Thu, Nov 17, 2022 at 01:44:12PM -0800, Eric Dumazet wrote:
> 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.

Indeed! Just tested and dropping this one still makes the test pass.

I believe this patch was papering over the issues fixed by the other
patches, so it stuck.

I will drop this one and move over to trying your suggestions for 2/3.

Thanks for taking a look,

 - Joel