[tip: sched/urgent] sched/core: Mitigate race cpus_share_cache()/update_top_cache_domain()

["tip-bot2 for Vincent Donnefort" <tip-bot2@xxxxxxxxxxxxx>]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     42dc938a590c96eeb429e1830123fef2366d9c80
Gitweb:        https://git.kernel.org/tip/42dc938a590c96eeb429e1830123fef2366d9c80
Author:        Vincent Donnefort <vincent.donnefort@xxxxxxx>
AuthorDate:    Thu, 04 Nov 2021 17:51:20 
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 11 Nov 2021 13:09:32 +01:00

sched/core: Mitigate race cpus_share_cache()/update_top_cache_domain()

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
---
 kernel/sched/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 523fd60..cec173a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3726,6 +3726,9 @@ 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);
 }