From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Jing-Ting Wu <jing-ting.wu@mediatek.com>,
        Vincent Donnefort <vincent.donnefort@arm.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 4.14 220/251] sched/core: Mitigate race cpus_share_cache()/update_top_cache_domain()
Date:   Wed, 24 Nov 2021 12:57:42 +0100
Message-Id: <20211124115717.918812279@linuxfoundation.org>
In-Reply-To: <20211124115710.214900256@linuxfoundation.org>
References: <20211124115710.214900256@linuxfoundation.org>
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From: Vincent Donnefort <vincent.donnefort@arm.com>

[ Upstream commit 42dc938a590c96eeb429e1830123fef2366d9c80 ]

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20211104175120.857087-1-vincent.donnefort@arm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/sched/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7cedada731c1b..544a1cb66d90d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1852,6 +1852,9 @@ out:
 
 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);
 }
 #endif /* CONFIG_SMP */
-- 
2.33.0