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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Waiman Long <longman@redhat.com>,
        Tejun Heo <tj@kernel.org>, Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.13 127/300] cgroup/cpuset: Fix a partition bug with hotplug
Date:   Mon, 13 Sep 2021 15:13:08 +0200
Message-Id: <20210913131113.688416893@linuxfoundation.org>
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From: Waiman Long <longman@redhat.com>

[ Upstream commit 15d428e6fe77fffc3f4fff923336036f5496ef17 ]

In cpuset_hotplug_workfn(), the detection of whether the cpu list
has been changed is done by comparing the effective cpus of the top
cpuset with the cpu_active_mask. However, in the rare case that just
all the CPUs in the subparts_cpus are offlined, the detection fails
and the partition states are not updated correctly. Fix it by forcing
the cpus_updated flag to true in this particular case.

Fixes: 4b842da276a8 ("cpuset: Make CPU hotplug work with partition")
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/cgroup/cpuset.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index adb5190c4429..592e9e37542f 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -3168,6 +3168,13 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
 	cpus_updated = !cpumask_equal(top_cpuset.effective_cpus, &new_cpus);
 	mems_updated = !nodes_equal(top_cpuset.effective_mems, new_mems);
 
+	/*
+	 * In the rare case that hotplug removes all the cpus in subparts_cpus,
+	 * we assumed that cpus are updated.
+	 */
+	if (!cpus_updated && top_cpuset.nr_subparts_cpus)
+		cpus_updated = true;
+
 	/* synchronize cpus_allowed to cpu_active_mask */
 	if (cpus_updated) {
 		spin_lock_irq(&callback_lock);
-- 
2.30.2