[PATCH-next v3 1/3] cgroup/cpuset: Correctly set cpus_updated in cpuset_hotplug_update_tasks()

[Waiman Long <longman@redhat.com>]

When cpuset_hotplug_update_tasks() is called with the conditions that
remote_partition_disable() has to be called, the cs->effective_cpus will
be updated such that it matches what is computed in new_cpus. In such
case, cpus_updated can be incorrectly set to false leading to stale CPU
afinity remained in the tasks not matching the new cpuset value. Fix that
by forcing cpus_updated to true after calling remote_partition_disable().

Fixes: 181c8e091aae ("cgroup/cpuset: Introduce remote partition")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/bug/linux-ab90f7e8-0d60-40ca-83e9-e405075b24d2
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 8a8a424a389f..862864b44960 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -4031,7 +4031,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 {
 	static cpumask_t new_cpus;
 	static nodemask_t new_mems;
-	bool cpus_updated;
+	bool cpus_updated = false;
 	bool mems_updated;
 	int partcmd = -1;
 	struct cpuset *parent;
@@ -4044,12 +4044,17 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 	if (!tmp || !cs->partition_root_state)
 		goto update_tasks;
 
+	/*
+	 * cpus_updated should be set to true when remote_partition_disable()
+	 * is called as cs->effective_cpus will be modified.
+	 */
 	if (is_remote_partition(cs) && (cpumask_empty(subpartitions_cpus) ||
 				       (cpumask_empty(&new_cpus) &&
 					partition_is_populated(cs, NULL)))) {
 		WRITE_ONCE(cs->prs_err, PERR_HOTPLUG);
 		remote_partition_disable(cs, tmp);
 		compute_effective_cpumask(&new_cpus, cs, parent);
+		cpus_updated = true;
 	}
 
 	/*
@@ -4082,7 +4087,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 	}
 
 update_tasks:
-	cpus_updated = !cpumask_equal(&new_cpus, cs->effective_cpus);
+	cpus_updated |= !cpumask_equal(&new_cpus, cs->effective_cpus);
 	mems_updated = !nodes_equal(new_mems, cs->effective_mems);
 	if (!cpus_updated && !mems_updated)
 		goto unlock;	/* Hotplug doesn't affect this cpuset */
-- 
2.56.0