Re: [PATCH-next v9 01/11] cgroup/cpuset: Make nr_deadline_tasks an atomic_t

[Ridong Chen <ridong.chen@linux.dev>]

On 6/30/2026 11:33 AM, Waiman Long wrote:
> The nr_deadline_tasks variable in the cpuset structure was introduced by
> commit 6c24849f5515 ("sched/cpuset: Keep track of SCHED_DEADLINE task
> in cpusets"). It is reported by sashiko [1] that nr_deadline_tasks
> can currently be modified by inc_dl_tasks_cs() under rq->lock and
> by cpuset_attach() under cpuset_mutex. So if both updates happen
> simultaneously, the nr_deadline_tasks variable can be corrupted leading
> to incorrect operations down the road.
> 
> Fix that by changing its type to atomic_t so that nr_deadline_tasks are
> always atomically updated.
> 
> [1] https://sashiko.dev/#/patchset/20260626181923.133658-1-longman%40redhat.comk
> 

Nit.

The link you provided has an extra 'k' at the end. Please remove it.

https://sashiko.dev/#/patchset/20260626181923.133658-1-longman%40redhat.com

> Fixes: 6c24849f5515 ("sched/cpuset: Keep track of SCHED_DEADLINE task in cpusets")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   kernel/cgroup/cpuset-internal.h |  2 +-
>   kernel/cgroup/cpuset.c          | 10 +++++-----
>   2 files changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
> index f7aaf01f7cd5..140700e5e236 100644
> --- a/kernel/cgroup/cpuset-internal.h
> +++ b/kernel/cgroup/cpuset-internal.h
> @@ -165,7 +165,7 @@ struct cpuset {
>   	 * number of SCHED_DEADLINE tasks attached to this cpuset, so that we
>   	 * know when to rebuild associated root domain bandwidth information.
>   	 */
> -	int nr_deadline_tasks;
> +	atomic_t nr_deadline_tasks;
>   	int nr_migrate_dl_tasks;
>   	/* DL bandwidth that needs destination reservation for this attach. */
>   	u64 sum_migrate_dl_bw;
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 49d8564d1a48..c22e55d798cf 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -222,14 +222,14 @@ void inc_dl_tasks_cs(struct task_struct *p)
>   {
>   	struct cpuset *cs = task_cs(p);
>   
> -	cs->nr_deadline_tasks++;
> +	atomic_inc(&cs->nr_deadline_tasks);
>   }
>   
>   void dec_dl_tasks_cs(struct task_struct *p)
>   {
>   	struct cpuset *cs = task_cs(p);
>   
> -	cs->nr_deadline_tasks--;
> +	atomic_dec(&cs->nr_deadline_tasks);
>   }
>   
>   static inline bool is_partition_valid(const struct cpuset *cs)
> @@ -918,7 +918,7 @@ static void dl_update_tasks_root_domain(struct cpuset *cs)
>   	struct css_task_iter it;
>   	struct task_struct *task;
>   
> -	if (cs->nr_deadline_tasks == 0)
> +	if (atomic_read(&cs->nr_deadline_tasks) == 0)
>   		return;
>   
>   	css_task_iter_start(&cs->css, 0, &it);
> @@ -3215,8 +3215,8 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>   	cs->old_mems_allowed = cpuset_attach_nodemask_to;
>   
>   	if (cs->nr_migrate_dl_tasks) {
> -		cs->nr_deadline_tasks += cs->nr_migrate_dl_tasks;
> -		oldcs->nr_deadline_tasks -= cs->nr_migrate_dl_tasks;
> +		atomic_add(cs->nr_migrate_dl_tasks, &cs->nr_deadline_tasks);
> +		atomic_sub(cs->nr_migrate_dl_tasks, &oldcs->nr_deadline_tasks);
>   		reset_migrate_dl_data(cs);
>   	}
>   

Reviewed-by: Ridong Chen <ridong.chen@linux.dev>

-- 
Best regards
Ridong