Re: [PATCH for-7.4-fixes] cgroup/cpuset: Remove redundant WARN_ON_ONCE() in remote_cpus_update()

[Waiman Long <longman@redhat.com>]

On 11/14/26 5:06 AM, Ridong Chen wrote:
> From: Ridong Chen <chenridong@xiaomi.com>
>
> remote_cpus_update() warns if the CPUs to be added to a remote partition
> intersect subpartitions_cpus, and then checks the very same condition
> right below to set PERR_NOCPUS and invalidate the partition:
>
> 	WARN_ON_ONCE(cpumask_intersects(tmp->addmask, subpartitions_cpus));
> 	if (!capable(CAP_SYS_ADMIN))
> 		WRITE_ONCE(cs->prs_err, PERR_ACCESS);
> 	else if (cpumask_intersects(tmp->addmask, subpartitions_cpus) ||
> 		 cpumask_subset(top_cpuset.effective_cpus, tmp->addmask))
> 		WRITE_ONCE(cs->prs_err, PERR_NOCPUS);
>
> Sashiko reported that the warning could be triggered by a write to a
> cpuset control file, before the CAP_SYS_ADMIN check, which would panic a
> system with panic_on_warn set.
>
> The warning was added by commit 86888c7bd117 ("cgroup/cpuset: Add
> warnings to catch inconsistency in exclusive CPUs") as an assertion that
> such CPUs have been filtered out before. No way to trigger it with the
> current interfaces has been found, as these CPUs are rejected when
> cpuset.cpus.exclusive is written. But the warning is redundant, as the
> conflict is handled right below anyway.
>
> Remove the warning and rely on the PERR_NOCPUS check, the same as
> remote_partition_enable() already does for this conflict.
>
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Link: https://sashiko.dev/#/patchset/20261114083124.1340140-1-ridong.chen%40linux.dev?part=6
> Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
> ---
>   kernel/cgroup/cpuset.c | 1 -
>   1 file changed, 1 deletion(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 7e374210c123..0be0d26c442b 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1833,7 +1833,6 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
>   	 * left in the top cpuset.
>   	 */
>   	if (adding) {
> -		WARN_ON_ONCE(cpumask_intersects(tmp->addmask, subpartitions_cpus));
>   		if (!capable(CAP_SYS_ADMIN))
>   			WRITE_ONCE(cs->prs_err, PERR_ACCESS);
>   		else if (cpumask_intersects(tmp->addmask, subpartitions_cpus) ||
>
> base-commit: e234ed1bb82c96a5643fd70784f16cf3d72eeed3
Reviewed-by: Waiman Long <longman@redhat.com>