Re: [PATCH] cpuset: relax locking on cpuset_node_allowed

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Apr 22, 2025 at 12:30:55AM -0400, 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

              ^^^^^^^^^^^^^

This should be callback_lock. rcu_read_lock() was intended for css
lifetime - which is ensured by css_get_e_css() - not a stable mask.

Otherwise looks good, makes sense to lampshade the lockless access.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>