[PATCH-next v2 2/5] 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
either remote_partition_disable() or update_parent_effective_cpumask()
has to be called, the cs->effective_cpus can 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 whenever one of these partition helpers is being called.

Fixes: 4b842da276a8a ("cpuset: Make CPU hotplug work with 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 | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 6074327c3477..3955d2d574cf 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -4002,7 +4002,7 @@ 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;
@@ -4015,12 +4015,18 @@ 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()
+	 * or update_parent_effective_cpumask() is called as cs->effective_cpus
+	 * may have been updated.
+	 */
 	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;
 	}
 
 	/*
@@ -4045,6 +4051,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 		partcmd = partcmd_update;
 
 	if (partcmd >= 0) {
+		cpus_updated = true;
 		update_parent_effective_cpumask(cs, partcmd, NULL, tmp);
 		if (is_partition_valid(cs) || (partcmd == partcmd_invalidate)) {
 			compute_effective_cpumask(&new_cpus, cs, parent);
@@ -4053,7 +4060,7 @@ 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