sched/core: Mitigate race cpus_share_cache()/update_top_cache_domain()
authorVincent Donnefort <vincent.donnefort@arm.com>
Thu, 4 Nov 2021 17:51:20 +0000 (17:51 +0000)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Fri, 26 Nov 2021 09:39:13 +0000 (10:39 +0100)
commit68fcb52b610c1853cadf3d135415555a128355f9
tree90221af532966698daa1f09acc930edfa6107cc8
parent91191d47af55658c386553ffa9ef3099701d150f
sched/core: Mitigate race cpus_share_cache()/update_top_cache_domain()

[ 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