linux-kernel - [PATCH v2] cgroup/cpuset: Change nr_deadline_tasks to an atomic

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Message-Id: <20231024141834.4073262-1-longman@redhat.com>
Date:   Tue, 24 Oct 2023 10:18:34 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        Juri Lelli <juri.lelli@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Qais Yousef <qyousef@...alina.io>, Hao Luo <haoluo@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Xia Fukun <xiafukun@...wei.com>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH v2] cgroup/cpuset: Change nr_deadline_tasks to an atomic_t value

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@...wei.com>
Signed-off-by: Waiman Long <longman@...hat.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