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sched_ext: idle: Recheck prev_cpu after narrowing allowed mask

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scx_select_cpu_dfl() narrows @allowed to @cpus_allowed & @p->cpus_ptr
when the BPF caller supplies a @cpus_allowed that differs from
@p->cpus_ptr and @p doesn't have full affinity. However,
@is_prev_allowed was computed against the original (wider)
@cpus_allowed, so the prev_cpu fast paths could pick a @prev_cpu that
is in @cpus_allowed but not in @p->cpus_ptr, violating the intended
invariant that the returned CPU is always usable by @p. The kernel
masks this via the SCX_EV_SELECT_CPU_FALLBACK fallback, but the
behavior contradicts the documented contract.

Move the @is_prev_allowed evaluation past the narrowing block so it
tests against the final @allowed mask.

Fixes: ee9a4e9279 ("sched_ext: idle: Properly handle invalid prev_cpu during idle selection")
Cc: stable@vger.kernel.org # v6.16+
Assisted-by: Claude <noreply@anthropic.com>
Signed-off-by: David Carlier <devnexen@gmail.com>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
David Carlier 2026-04-30 10:27:47 +01:00 committed by Tejun Heo
parent ff9eda4ea9
commit b34c82777a
1 changed files with 6 additions and 6 deletions
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12
kernel/sched/ext_idle.c
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@ -465,12 +465,6 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
preempt_disable();
/*
* Check whether @prev_cpu is still within the allowed set. If not,
* we can still try selecting a nearby CPU.
*/
is_prev_allowed = cpumask_test_cpu(prev_cpu, allowed);
/*
* Determine the subset of CPUs usable by @p within @cpus_allowed.
*/
@ -487,6 +481,12 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
}
}
/*
* Check whether @prev_cpu is still within the allowed set. If not,
* we can still try selecting a nearby CPU.
*/
is_prev_allowed = cpumask_test_cpu(prev_cpu, allowed);
/*
* This is necessary to protect llc_cpus.
*/