releases/5.10.99/cgroup-cpuset-fix-suspicious-rcu-usage-lockdep-warning.patch

From 2bdfd2825c9662463371e6691b1a794e97fa36b4 Mon Sep 17 00:00:00 2001
From: Waiman Long <longman@redhat.com>
Date: Wed, 2 Feb 2022 22:31:03 -0500
Subject: cgroup/cpuset: Fix "suspicious RCU usage" lockdep warning

From: Waiman Long <longman@redhat.com>

commit 2bdfd2825c9662463371e6691b1a794e97fa36b4 upstream.

It was found that a "suspicious RCU usage" lockdep warning was issued
with the rcu_read_lock() call in update_sibling_cpumasks().  It is
because the update_cpumasks_hier() function may sleep. So we have
to release the RCU lock, call update_cpumasks_hier() and reacquire
it afterward.

Also add a percpu_rwsem_assert_held() in update_sibling_cpumasks()
instead of stating that in the comment.

Fixes: 4716909cc5c5 ("cpuset: Track cpusets that use parent's effective_cpus")
Signed-off-by: Waiman Long <longman@redhat.com>
Tested-by: Phil Auld <pauld@redhat.com>
Reviewed-by: Phil Auld <pauld@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 kernel/cgroup/cpuset.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1481,10 +1481,15 @@ static void update_sibling_cpumasks(stru
 	struct cpuset *sibling;
 	struct cgroup_subsys_state *pos_css;
 
+	percpu_rwsem_assert_held(&cpuset_rwsem);
+
 	/*
 	 * Check all its siblings and call update_cpumasks_hier()
 	 * if their use_parent_ecpus flag is set in order for them
 	 * to use the right effective_cpus value.
+	 *
+	 * The update_cpumasks_hier() function may sleep. So we have to
+	 * release the RCU read lock before calling it.
 	 */
 	rcu_read_lock();
 	cpuset_for_each_child(sibling, pos_css, parent) {
@@ -1492,8 +1497,13 @@ static void update_sibling_cpumasks(stru
 			continue;
 		if (!sibling->use_parent_ecpus)
 			continue;
+		if (!css_tryget_online(&sibling->css))
+			continue;
 
+		rcu_read_unlock();
 		update_cpumasks_hier(sibling, tmp);
+		rcu_read_lock();
+		css_put(&sibling->css);
 	}
 	rcu_read_unlock();
 }