Re: [PATCH v2 3/5] scftorture: Protect the initial affinity check from hotplug

[Paul E. McKenney]

On Sun, Nov 08, 2026 at 07:02:39PM -0500, Joel Fernandes wrote:
> An invoker binds to its target CPU, sleeps for the holdoff interval, and
> then warns if it is running elsewhere. CPU hotplug can offline the target
> during that sleep and legitimately move the invoker. A built-in test with
> matching worker and hotplug holdoffs reproduces this warning.
> 
> Check the initial binding while holding cpus_read_lock(), before sleeping.
> If the target is already offline, let the existing offline/rebind path
> handle it instead of warning about an impossible initial binding. Release
> the read lock before the holdoff and test loop so hotplug remains tested.
> 
> Signed-off-by: Joel Fernandes <joelagnelf@xxxxxxxxxx>

Good catch!

Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> ---
>  kernel/scftorture.c | 19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/scftorture.c b/kernel/scftorture.c
> index ff9a8271a464..64ee2c5cf51c 100644
> --- a/kernel/scftorture.c
> +++ b/kernel/scftorture.c
> @@ -479,19 +479,24 @@ static int scftorture_invoker(void *arg)
>  
>  	VERBOSE_SCFTORTOUT("scftorture_invoker %d: task started", scfp->cpu);
>  	cpu = scfp->cpu % nr_cpu_ids;
> -	WARN_ON_ONCE(set_cpus_allowed_ptr(current, cpumask_of(cpu)));
> +	cpus_read_lock();
> +	if (cpu_online(cpu)) {
> +		WARN_ON_ONCE(set_cpus_allowed_ptr(current, cpumask_of(cpu)));
> +		/* Check affinity before hotplug can move this task away. */
> +		curcpu = raw_smp_processor_id();
> +		WARN_ONCE(curcpu != cpu,
> +			  "%s: Wanted CPU %d, running on %d, nr_cpu_ids = %d\n",
> +			  __func__, scfp->cpu, curcpu, nr_cpu_ids);
> +	} else {
> +		was_offline = true;
> +	}
> +	cpus_read_unlock();
>  	set_user_nice(current, MAX_NICE);
>  	if (holdoff)
>  		schedule_timeout_interruptible(holdoff * HZ);
>  
>  	VERBOSE_SCFTORTOUT("scftorture_invoker %d: Waiting for all SCF torturers from cpu %d", scfp->cpu, raw_smp_processor_id());
>  
> -	// Make sure that the CPU is affinitized appropriately during testing.
> -	curcpu = raw_smp_processor_id();
> -	WARN_ONCE(curcpu != cpu,
> -		  "%s: Wanted CPU %d, running on %d, nr_cpu_ids = %d\n",
> -		  __func__, scfp->cpu, curcpu, nr_cpu_ids);
> -
>  	if (atomic_dec_return(&n_started))
>  		while (atomic_read_acquire(&n_started)) {
>  			if (torture_must_stop()) {
> -- 
> 2.54.0
>