Re: [PATCH] cpuset: relax locking on cpuset_node_allowed

[Waiman Long <llong@redhat.com>]

On 4/22/25 12:30 AM, Gregory Price wrote:
> The cgroup_get_e_css reference protects the css->effective_mems, and
> calls of this interface would be subject to the same race conditions
> associated with a non-atomic access to cs->effective_mems.
>
> So while this interface cannot make strong guarantees of correctness,
> it can therefore avoid taking a global or rcu_read_lock for performance.
>
> Drop the rcu_read_lock from cpuset_node_allowed.
>
> Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
> Suggested-by: Waiman Long <longman@redhat.com>
> Signed-off-by: Gregory Price <gourry@gourry.net>
> ---
>   kernel/cgroup/cpuset.c | 14 ++++++++++++--
>   1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index c52348bfd5db..1dc41758c62c 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -4181,10 +4181,20 @@ bool cpuset_node_allowed(struct cgroup *cgroup, int nid)
>   	if (!css)
>   		return true;
>   
> +	/*
> +	 * Normally, accessing effective_mems would require the cpuset_mutex
> +	 * or RCU read lock - but node_isset is atomic and the reference
> +	 * taken via cgroup_get_e_css is sufficient to protect css.
> +	 *
> +	 * Since this interface is intended for use by migration paths, we
> +	 * relax locking here to avoid taking global locks - while accepting
> +	 * there may be rare scenarios where the result may be innaccurate.
> +	 *
> +	 * Reclaim and migration are subject to these same race conditions, and
> +	 * cannot make strong isolation guarantees, so this is acceptable.
> +	 */
>   	cs = container_of(css, struct cpuset, css);
> -	rcu_read_lock();
>   	allowed = node_isset(nid, cs->effective_mems);
> -	rcu_read_unlock();
>   	css_put(css);
>   	return allowed;
>   }

Except for mislabeling RCU read lock instead of callback_lock as pointed 
out by Johannes, the change looks good to me.

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