From: Vincent Guittot <vincent.guittot@linaro.org>
To: Vincent Donnefort <Vincent.Donnefort@arm.com>
Cc: peterz@infradead.org, mingo@redhat.com,
linux-kernel@vger.kernel.org, dietmar.eggemann@arm.com,
Valentin.Schneider@arm.com, jing-ting.wu@mediatek.com
Subject: Re: [PATCH] sched/core: Mitigate race cpus_share_cache()/update_top_cache_domain()
Date: Fri, 5 Nov 2021 16:08:33 +0100 [thread overview]
Message-ID: <CAKfTPtAQTCqhbaW90EiyerPM2x-==cX3Xodq+nAymVK_U=45vA@mail.gmail.com> (raw)
In-Reply-To: <20211104175120.857087-1-vincent.donnefort@arm.com>
On Thu, 4 Nov 2021 at 18:52, Vincent Donnefort
<vincent.donnefort@arm.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@mediatek.com>
> Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
> Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.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
>
next prev parent reply other threads:[~2021-11-05 15:08 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-04 17:51 [PATCH] sched/core: Mitigate race cpus_share_cache()/update_top_cache_domain() Vincent Donnefort
2021-11-05 15:08 ` Vincent Guittot [this message]
2021-11-08 11:19 ` Peter Zijlstra
2021-11-11 12:22 ` [tip: sched/urgent] " tip-bot2 for Vincent Donnefort
2021-11-15 10:46 ` [PATCH] " Shinichiro Kawasaki
2021-11-15 18:33 ` Valentin Schneider
2021-11-16 8:02 ` Vincent Guittot
2021-11-16 8:45 ` Shinichiro Kawasaki
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