[PATCH v2] cgroup/cpuset: Change nr_deadline_tasks to an atomic_t value

Waiman Long
2023-10-24T14:19:14Z

The nr_deadline_tasks field in cpuset structure was introduced by
commit 6c24849f5515 ("sched/cpuset: Keep track of SCHED_DEADLINE task
in cpusets"). Unlike nr_migrate_dl_tasks which is only modified under
cpuset_mutex, nr_deadline_tasks can be updated under two different
locks - cpuset_mutex in most cases or css_set_lock in cgroup_exit(). As
a result, data races can happen leading to incorrect nr_deadline_tasks
value.

Since it is not practical to somehow take cpuset_mutex in cgroup_exit(),
the easy way out to avoid this possible race condition is by making
nr_deadline_tasks an atomic_t value.

Fixes: 6c24849f5515 ("sched/cpuset: Keep track of SCHED_DEADLINE task in cpusets")
Reported-by: Xia Fukun <xiafukun@huawei.com>
Signed-off-by: Waiman Long <longman@redhat.com>
---
kernel/cgroup/cpuset.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 58ec88efa4f8..3f3da468f058 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -174,7 +174,7 @@ 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;
u64 sum_migrate_dl_bw;

@@ -234,14 +234,14 @@ 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);
}

/* bits in struct cpuset flags field */
@@ -1071,7 +1071,7 @@ 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);
@@ -2721,8 +2721,8 @@ 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);
}

--
2.39.3