linux-kernel - [PATCH 2/3] cgroup/cpuset.c: Fix a partition error with CPU hotplug

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Message-ID: <20250806172430.1155133-3-longman@redhat.com>
Date: Wed,  6 Aug 2025 13:24:29 -0400
From: Waiman Long <longman@...hat.com>
To: Tejun Heo <tj@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>
Cc: cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Waiman Long <longman@...hat.com>
Subject: [PATCH 2/3] cgroup/cpuset.c: Fix a partition error with CPU hotplug

It was found during testing that an invalid leaf partition with an
empty effective exclusive CPU list can become a valid empty partition
with no CPU afer an offline/online operation of an unrelated CPU. An
empty partition root is allowed in the special case that it has no
task in its cgroup and has distributed out all its CPUs to its child
partitions. That is certainly not the case here.

The problem is in the cpumask_subsets() test in the hotplug case
(update with no new mask) of update_parent_effective_cpumask() as it
also returns true if the effective exclusive CPU list is empty. Fix that
by addding the cpumask_empty() test to root out this exception case.
Also add the cpumask_empty() test in cpuset_hotplug_update_tasks()
to avoid calling update_parent_effective_cpumask() for this special case.

Fixes: 0c7f293efc87 ("cgroup/cpuset: Add cpuset.cpus.exclusive.effective for v2")
Signed-off-by: Waiman Long <longman@...hat.com>
---
 kernel/cgroup/cpuset.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index bf149246e001..d993e058a663 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1843,7 +1843,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
 			if (is_partition_valid(cs))
 				adding = cpumask_and(tmp->addmask,
 						xcpus, parent->effective_xcpus);
-		} else if (is_partition_invalid(cs) &&
+		} else if (is_partition_invalid(cs) && !cpumask_empty(xcpus) &&
 			   cpumask_subset(xcpus, parent->effective_xcpus)) {
 			struct cgroup_subsys_state *css;
 			struct cpuset *child;
@@ -3870,9 +3870,10 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 		partcmd = partcmd_invalidate;
 	/*
 	 * On the other hand, an invalid partition root may be transitioned
-	 * back to a regular one.
+	 * back to a regular one with a non-empty effective xcpus.
 	 */
-	else if (is_partition_valid(parent) && is_partition_invalid(cs))
+	else if (is_partition_valid(parent) && is_partition_invalid(cs) &&
+		 !cpumask_empty(cs->effective_xcpus))
 		partcmd = partcmd_update;
 
 	if (partcmd >= 0) {
-- 
2.50.0