cgroup/cpuset: Make nr_deadline_tasks an atomic_t

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. This fix patch is a low hanging fruit.
It can handle some of the races between a concurrent sched_setscheduler()
and cpuset_can_attach()/cpuset_attach() calls, but not all of them like
the other issue raised by sashiko [2]. This will be handled hopefully
in a future follow up patch.

[1] https://sashiko.dev/#/patchset/20260626181923.133658-1-longman%40redhat.com
[2] https://sashiko.dev/#/patchset/20260630033344.352702-1-longman%40redhat.com

Fixes: 6c24849f5515 ("sched/cpuset: Keep track of SCHED_DEADLINE task in cpusets")
Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

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 e53f35e2726f..7c54289edf40 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 @@ out:
 	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);
 	}