linux-kernel - Re: [PATCH] sched/core: Mitigate race cpus_share_cache()/update_top_cache

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Date:   Mon, 8 Nov 2021 12:19:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Vincent Donnefort <Vincent.Donnefort@....com>, mingo@...hat.com,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        Valentin.Schneider@....com, jing-ting.wu@...iatek.com
Subject: Re: [PATCH] sched/core: Mitigate race
 cpus_share_cache()/update_top_cache_domain()

On Fri, Nov 05, 2021 at 04:08:33PM +0100, Vincent Guittot wrote:
> On Thu, 4 Nov 2021 at 18:52, Vincent Donnefort
> <vincent.donnefort@....com> wrote:
> >
> > Nothing protects the access to the per_cpu variable sd_llc_id. When testing
> > the same CPU (i.e. this_cpu == that_cpu), a race condition exists with
> > update_top_cache_domain(). One scenario being:
> >
> >               CPU1                            CPU2
> >   ==================================================================
> >
> >   per_cpu(sd_llc_id, CPUX) => 0
> >                                     partition_sched_domains_locked()
> >                                       detach_destroy_domains()
> >   cpus_share_cache(CPUX, CPUX)          update_top_cache_domain(CPUX)
> >     per_cpu(sd_llc_id, CPUX) => 0
> >                                           per_cpu(sd_llc_id, CPUX) = CPUX
> >     per_cpu(sd_llc_id, CPUX) => CPUX
> >     return false
> >
> > ttwu_queue_cond() wouldn't catch smp_processor_id() == cpu and the result
> > is a warning triggered from ttwu_queue_wakelist().
> >
> > Avoid a such race in cpus_share_cache() by always returning true when
> > this_cpu == that_cpu.
> >
> > Fixes: 518cd6234178 ("sched: Only queue remote wakeups when crossing cache boundaries")
> > Reported-by: Jing-Ting Wu <jing-ting.wu@...iatek.com>
> > Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
> > Reviewed-by: Valentin Schneider <valentin.schneider@....com>
> 
>  Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
> 
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index f2611b9cf503..f5ca15cdcff4 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3726,6 +3726,9 @@ void wake_up_if_idle(int cpu)
> >
> >  bool cpus_share_cache(int this_cpu, int that_cpu)
> >  {
> > +       if (this_cpu == that_cpu)
> > +               return true;
> > +
> >         return per_cpu(sd_llc_id, this_cpu) == per_cpu(sd_llc_id, that_cpu);
> >  }

Blergh, that's annoying. Thanks guys, picked it up for /urgent