linux-kernel - [PATCH -next] cpuset: treate root invalid trialcs as exclusive

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Message-Id: <20251115093140.1121329-1-chenridong@huaweicloud.com>
Date: Sat, 15 Nov 2025 09:31:40 +0000
From: Chen Ridong <chenridong@...weicloud.com>
To: longman@...hat.com,
	tj@...nel.org,
	hannes@...xchg.org,
	mkoutny@...e.com
Cc: cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH -next] cpuset: treate root invalid trialcs as exclusive

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

A test scenario revealed inconsistent results based on operation order:
Scenario 1:
	#cd /sys/fs/cgroup/
	#mkdir A1
	#mkdir B1
	#echo 1-2 > B1/cpuset.cpus
	#echo 0-1 > A1/cpuset.cpus
	#echo root > A1/cpuset.cpus.partition
	#cat A1/cpuset.cpus.partition
	root invalid (Cpu list in cpuset.cpus not exclusive)

Scenario 2:
	#cd /sys/fs/cgroup/
	#mkdir A1
	#mkdir B1
	#echo 1-2 > B1/cpuset.cpus
	#echo root > A1/cpuset.cpus.partition
	#echo 0-1 > A1/cpuset.cpus
	#cat A1/cpuset.cpus.partition
	root

The second scenario produces an unexpected result: A1 should be marked
as invalid but is incorrectly recognized as valid. This occurs because
when validate_change is invoked, A1 (in root-invalid state) may
automatically transition to a valid partition, with non-exclusive state
checks against siblings, leading to incorrect validation.

To fix this inconsistency, treat trialcs in root-invalid state as exclusive
during validation and set the corresponding exclusive flags, ensuring
consistent behavior regardless of operation order.

Signed-off-by: Chen Ridong <chenridong@...wei.com>
---
 kernel/cgroup/cpuset.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index daf813386260..a189f356b5f1 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2526,6 +2526,18 @@ static void partition_cpus_change(struct cpuset *cs, struct cpuset *trialcs,
 	}
 }
 
+static int init_trialcs(struct cpuset *cs, struct cpuset *trialcs)
+{
+	trialcs->prs_err = PERR_NONE;
+	/*
+	 * If partition_root_state != 0, it may automatically change to a partition,
+	 * Therefore, we should treat trialcs as exclusive during validation
+	 */
+	if (trialcs->partition_root_state)
+		set_bit(CS_CPU_EXCLUSIVE, &trialcs->flags);
+	return compute_trialcs_excpus(trialcs, cs);
+}
+
 /**
  * update_cpumask - update the cpus_allowed mask of a cpuset and all tasks in it
  * @cs: the cpuset to consider
@@ -2551,9 +2563,7 @@ static int update_cpumask(struct cpuset *cs, struct cpuset *trialcs,
 	if (alloc_tmpmasks(&tmp))
 		return -ENOMEM;
 
-	compute_trialcs_excpus(trialcs, cs);
-	trialcs->prs_err = PERR_NONE;
-
+	init_trialcs(cs, trialcs);
 	retval = cpus_allowed_validate_change(cs, trialcs, &tmp);
 	if (retval < 0)
 		goto out_free;
@@ -2612,7 +2622,7 @@ static int update_exclusive_cpumask(struct cpuset *cs, struct cpuset *trialcs,
 	 * Reject the change if there is exclusive CPUs conflict with
 	 * the siblings.
 	 */
-	if (compute_trialcs_excpus(trialcs, cs))
+	if (init_trialcs(cs, trialcs))
 		return -EINVAL;
 
 	/*
@@ -2628,7 +2638,6 @@ static int update_exclusive_cpumask(struct cpuset *cs, struct cpuset *trialcs,
 	if (alloc_tmpmasks(&tmp))
 		return -ENOMEM;
 
-	trialcs->prs_err = PERR_NONE;
 	partition_cpus_change(cs, trialcs, &tmp);
 
 	spin_lock_irq(&callback_lock);
-- 
2.34.1