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

[Ridong Chen]

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 <long...@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.c...@linux.dev>

--
Best regards
Ridong