linux-kernel - [PATCH -next RFC 14/16] cpuset: fix isolcpus stay in root when isolated partition changes to root

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Message-Id: <20250928071306.3797436-15-chenridong@huaweicloud.com>
Date: Sun, 28 Sep 2025 07:13:04 +0000
From: Chen Ridong <chenridong@...weicloud.com>
To: longman@...hat.com,
	tj@...nel.org,
	hannes@...xchg.org,
	mkoutny@...e.com
Cc: cgups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	lujialin4@...wei.com,
	chenridong@...wei.com
Subject: [PATCH -next RFC 14/16] cpuset: fix isolcpus stay in root when isolated partition changes to root

From: Chen Ridong <chenridong@...wei.com>

A bug was detected with the following steps:

  # cd /sys/fs/cgroup/
  # mkdir test
  # echo 9 > test/cpuset.cpus
  # echo isolated > test/cpuset.cpus.partition
  # cat test/cpuset.cpus.partition
  isolated
  # cat test/cpuset.cpus
  9
  # echo root > test/cpuset.cpus.partition
  # cat test/cpuset.cpus
  9
  # cat test/cpuset.cpus.partition
  root

CPU 9 was initially placed in an isolated partition. When the partition
type is changed from isolated to root, CPU 9 remains in what becomes a
valid root partition. This violates the rule that isolcpus can only be
assigned to isolated partitions.

Fix the issue by re-enabling partition validation, which performs
comprehensive partition error checking. In the scenario described above,
this change causes the operation to fail with housekeeping conflicts,
preventing the invalid configuration.

Additionally, when enable a local partition, the warning for tmp->addmask
not being a subset of parent's effective CPUs was removed. This warning was
triggered during local partition re-enablement because the CPUs were
already added to exclusive_cpus during the previous enable operation. The
subset check is not applicable in this re-enablement scenario.

Fixes: f28e22441f35 ("cgroup/cpuset: Add a new isolated cpus.partition type")
Signed-off-by: Chen Ridong <chenridong@...wei.com>
---
 kernel/cgroup/cpuset.c | 35 +++++++++--------------------------
 1 file changed, 9 insertions(+), 26 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 20288dbd6ccf..2aaa688c596f 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1873,6 +1873,7 @@ static int local_partition_enable(struct cpuset *cs,
 {
 	struct cpuset *parent = parent_cs(cs);
 	enum prs_errcode part_error;
+	bool cpumask_updated = false;
 
 	lockdep_assert_held(&cpuset_mutex);
 	WARN_ON_ONCE(is_remote_partition(cs));	/* For local partition only */
@@ -1899,22 +1900,14 @@ static int local_partition_enable(struct cpuset *cs,
 	if (part_error)
 		return part_error;
 
-	/*
-	 * This function will only be called when all the preliminary
-	 * checks have passed. At this point, the following condition
-	 * should hold.
-	 *
-	 * (cs->effective_xcpus & cpu_active_mask) ⊆ parent->effective_cpus
-	 *
-	 * Warn if it is not the case.
-	 * addmask is used as temporary variable.
-	 */
-	cpumask_and(tmp->addmask, tmp->new_cpus, cpu_active_mask);
-	WARN_ON_ONCE(!cpumask_subset(tmp->addmask, parent->effective_cpus));
+	cpumask_updated = cpumask_andnot(tmp->addmask, tmp->new_cpus,
+					 parent->effective_cpus);
 	partition_enable(cs, parent, new_prs, tmp->new_cpus);
 
-	cpuset_update_tasks_cpumask(parent, tmp->addmask);
-	update_sibling_cpumasks(parent, cs, tmp);
+	if (cpumask_updated) {
+		cpuset_update_tasks_cpumask(parent, tmp->addmask);
+		update_sibling_cpumasks(parent, cs, tmp);
+	}
 	return 0;
 }
 
@@ -2902,7 +2895,6 @@ static int update_prstate(struct cpuset *cs, int new_prs)
 	int err = PERR_NONE, old_prs = cs->partition_root_state;
 	struct cpuset *parent = parent_cs(cs);
 	struct tmpmasks tmpmask;
-	bool isolcpus_updated = false;
 
 	if (old_prs == new_prs)
 		return 0;
@@ -2920,7 +2912,7 @@ static int update_prstate(struct cpuset *cs, int new_prs)
 	if (err)
 		goto out;
 
-	if (!old_prs) {
+	if (new_prs > 0) {
 		/*
 		 * cpus_allowed and exclusive_cpus cannot be both empty.
 		 */
@@ -2950,12 +2942,6 @@ static int update_prstate(struct cpuset *cs, int new_prs)
 			err = local_partition_enable(cs, new_prs, &tmpmask);
 		else
 			err = remote_partition_enable(cs, new_prs, &tmpmask);
-	} else if (old_prs && new_prs) {
-		/*
-		 * A change in load balance state only, no change in cpumasks.
-		 * Need to update isolated_cpus.
-		 */
-		isolcpus_updated = true;
 	} else {
 		/*
 		 * Switching back to member is always allowed even if it
@@ -2985,16 +2971,13 @@ static int update_prstate(struct cpuset *cs, int new_prs)
 	WRITE_ONCE(cs->prs_err, err);
 	if (!is_partition_valid(cs))
 		reset_partition_data(cs);
-	else if (isolcpus_updated)
-		isolated_cpus_update(old_prs, new_prs, cs->effective_xcpus);
 	spin_unlock_irq(&callback_lock);
-	update_unbound_workqueue_cpumask(isolcpus_updated);
 
 	/* Force update if switching back to member & update effective_xcpus */
 	update_cpumasks_hier(cs, &tmpmask, !new_prs);
 
 	/* A newly created partition must have effective_xcpus set */
-	WARN_ON_ONCE(!old_prs && (new_prs > 0)
+	WARN_ON_ONCE(!old_prs && (cs->partition_root_state > 0)
 			      && cpumask_empty(cs->effective_xcpus));
 
 	/* Update sched domains and load balance flag */
-- 
2.34.1