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Currently, the code uses > __cpumask_clear_cpu() to clear a CPU from the mask. Because this is > a non-atomic read-modify-write operation, concurrent modifications to > different bits within the same memory word can lead to data races and > lost updates. > > Fix this by replacing __cpumask_clear_cpu() with the atomic > cpumask_clear_cpu(). I think the atomic clear makes sense here, but, as sashiko also pointed out, it does not fully address the race, because idle_smts is also modified by the non-atomic cpumask_andnot() below and cpumask_or() in update_builtin_idle(). > > Fixes: 48849271e661 ("sched_ext: idle: Per-node idle cpumasks") And the race existed way before this commit, the idle SMT tracking has been always documented as racy and self-correcting. This change may still be a best-effort improvement, but the commit message should describe it in this way. Did you notice any improvements/benefits with some workloads with this patch applied? Thanks, -Andrea > Signed-off-by: Michal Blaszczyk > --- > kernel/sched/ext/idle.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/sched/ext/idle.c b/kernel/sched/ext/idle.c > index d2973fb3af6d..8985b48c83a5 100644 > --- a/kernel/sched/ext/idle.c > +++ b/kernel/sched/ext/idle.c > @@ -104,7 +104,7 @@ static bool scx_idle_test_and_clear_cpu(int cpu) > if (cpumask_intersects(smt, idle_smts)) > cpumask_andnot(idle_smts, idle_smts, smt); > else if (cpumask_test_cpu(cpu, idle_smts)) > - __cpumask_clear_cpu(cpu, idle_smts); > + cpumask_clear_cpu(cpu, idle_smts); > } > > return cpumask_test_and_clear_cpu(cpu, idle_cpus); > -- > 2.55.0.897.gb25b4bd76c-goog >