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

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Date:   Fri, 5 Nov 2021 16:08:33 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Vincent Donnefort <Vincent.Donnefort@....com>
Cc:     peterz@...radead.org, 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 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);
>  }
>
> --
> 2.25.1
>