This is a note to let you know that I've just added the patch titled sched/core: Mitigate race cpus_share_cache()/update_top_cache_domain() to the 4.14-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: sched-core-mitigate-race-cpus_share_cache-update_top.patch and it can be found in the queue-4.14 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit bc334949276f9723c1c849d3582a8aebb216522c Author: Vincent Donnefort <vincent.donnefort@xxxxxxx> Date: Thu Nov 4 17:51:20 2021 +0000 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@xxxxxxxxxxxx> Signed-off-by: Vincent Donnefort <vincent.donnefort@xxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx> Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx> Link: https://lore.kernel.org/r/20211104175120.857087-1-vincent.donnefort@xxxxxxx Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> 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 @@ 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); } #endif /* CONFIG_SMP */