Re: [PATCH] sched: fix warning in sched_setaffinity

[Waiman Long <longman@redhat.com>]

On 8/29/24 18:04, Josh Don wrote:
> Commit 8f9ea86fdf99b added some logic to sched_setaffinity that included
> a WARN when a per-task affinity assignment races with a cpuset update.
>
> Specifically, we can have a race where a cpuset update results in the
> task affinity no longer being a subset of the cpuset. That's fine; we
> have a fallback to instead use the cpuset mask. However, we have a WARN
> set up that will trigger if the cpuset mask has no overlap at all with
> the requested task affinity. This shouldn't be a warning condition; its
> trivial to create this condition.
>
> Reproduced the warning by the following setup:
>
> - $PID inside a cpuset cgroup
> - another thread repeatedly switching the cpuset cpus from 1-2 to just 1
> - another thread repeatedly setting the $PID affinity (via taskset) to 2
>
> Fixes: 8f9ea86fdf99b ("sched: Always preserve the user requested cpumask")
> Signed-off-by: Josh Don <joshdon@google.com>
> ---
>   kernel/sched/syscalls.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index 4fae3cf25a3a..3a88f7c0cb69 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -1321,7 +1321,7 @@ int __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx)
>   			bool empty = !cpumask_and(new_mask, new_mask,
>   						  ctx->user_mask);
>   
> -			if (WARN_ON_ONCE(empty))
> +			if (empty)
>   				cpumask_copy(new_mask, cpus_allowed);
>   		}
>   		__set_cpus_allowed_ptr(p, ctx);

Taking out the WARN_ON_ONCE() should be fine.

Acked-by: Waiman Long <longman@redhat.com>