Re: [tip:sched/core] [sched] b079d93796: WARNING:possible_recursive_locking_detected_migration_is_trying_to_acquire_lock:at:set_cpus_allowed_force_but_task_is_already_holding_lock:at:cpu_stopper_thread

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Oct 27, 2025 at 12:01:33PM +0100, Peter Zijlstra wrote:

Could someone confirm this fixes the problem?

> ---
> Subject: sched: Fix the do_set_cpus_allowed() locking fix
> 
> Commit abfc01077df6 ("sched: Fix do_set_cpus_allowed() locking")
> overlooked that __balance_push_cpu_stop() calls select_fallback_rq()
> with rq->lock held. This makes that set_cpus_allowed_force() will
> recursively take rq->lock and the machine locks up.
> 
> Run select_fallback_rq() earlier, without holding rq->lock. This opens
> up a race window where a task could get migrated out from under us, but
> that is harmless, we want the task migrated.
> 
> select_fallback_rq() itself will not be subject to concurrency as it
> will be fully serialized by p->pi_lock, so there is no chance of
> set_cpus_allowed_force() getting called with different arguments and
> selecting different fallback CPUs for one task.
> 
> Fixes: abfc01077df6 ("sched: Fix do_set_cpus_allowed() locking")
> Reported-by: Jan Polensky <japo@linux.ibm.com>
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Closes: https://lore.kernel.org/oe-lkp/202510271206.24495a68-lkp@intel.com
> ---
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 1842285eac1e..67b5f2faab36 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8044,18 +8044,15 @@ static int __balance_push_cpu_stop(void *arg)
>  	struct rq_flags rf;
>  	int cpu;
>  
> -	raw_spin_lock_irq(&p->pi_lock);
> -	rq_lock(rq, &rf);
> -
> -	update_rq_clock(rq);
> -
> -	if (task_rq(p) == rq && task_on_rq_queued(p)) {
> +	scoped_guard (raw_spinlock_irq, &p->pi_lock) {
>  		cpu = select_fallback_rq(rq->cpu, p);
> -		rq = __migrate_task(rq, &rf, p, cpu);
> -	}
>  
> -	rq_unlock(rq, &rf);
> -	raw_spin_unlock_irq(&p->pi_lock);
> +		rq_lock(rq, &rf);
> +		update_rq_clock(rq);
> +		if (task_rq(p) == rq && task_on_rq_queued(p))
> +			rq = __migrate_task(rq, &rf, p, cpu);
> +		rq_unlock(rq, &rf);
> +	}
>  
>  	put_task_struct(p);
>