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cgroup/cpuset: Make nr_deadline_tasks an atomic_t

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The nr_deadline_tasks variable in the cpuset structure was introduced by
commit 6c24849f55 ("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: 6c24849f55 ("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>
This commit is contained in:
Waiman Long 2026-07-02 17:47:47 -04:00 committed by Tejun Heo
parent e9d189aa4b
commit 95220e1f18
2 changed files with 6 additions and 6 deletions
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2
kernel/cgroup/cpuset-internal.h
View File

@ -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;

10
kernel/cgroup/cpuset.c
View File

@ -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);
}