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From:   Tim Van Patten <timvp@chromium.org>
To:     LKML <linux-kernel@vger.kernel.org>
Cc:     markhas@google.com, Tim Van Patten <timvp@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        cgroups@vger.kernel.org
Subject: [PATCH] cgroup_freezer: cgroup_freezing: Check if not frozen
Date:   Wed, 15 Nov 2023 09:20:43 -0700
Message-ID: <20231115162054.2896748-1-timvp@chromium.org>
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From: Tim Van Patten <timvp@google.com>

__thaw_task() was recently updated to warn if the task being thawed was
part of a freezer cgroup that is still currently freezing:

	void __thaw_task(struct task_struct *p)
	{
	...
		if (WARN_ON_ONCE(freezing(p)))
			goto unlock;

This has exposed a bug in cgroup1 freezing where when CGROUP_FROZEN is
asserted, the CGROUP_FREEZING bits are not also cleared at the same
time. Meaning, when a cgroup is marked FROZEN it continues to be marked
FREEZING as well. This causes the WARNING to trigger, because
cgroup_freezing() thinks the cgroup is still freezing.

There are two ways to fix this:

1. Whenever FROZEN is set, clear FREEZING for the cgroup and all
children cgroups.
2. Update cgroup_freezing() to also verify that FROZEN is not set.

This patch implements option (2), since it's smaller and more
straightforward.

Signed-off-by: Tim Van Patten <timvp@google.com>
---

 kernel/cgroup/legacy_freezer.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/legacy_freezer.c b/kernel/cgroup/legacy_freezer.c
index 122dacb3a443..66d1708042a7 100644
--- a/kernel/cgroup/legacy_freezer.c
+++ b/kernel/cgroup/legacy_freezer.c
@@ -66,9 +66,15 @@ static struct freezer *parent_freezer(struct freezer *freezer)
 bool cgroup_freezing(struct task_struct *task)
 {
 	bool ret;
+	unsigned int state;
 
 	rcu_read_lock();
-	ret = task_freezer(task)->state & CGROUP_FREEZING;
+	/* Check if the cgroup is still FREEZING, but not FROZEN. The extra
+	 * !FROZEN check is required, because the FREEZING bit is not cleared
+	 * when the state FROZEN is reached.
+	 */
+	state = task_freezer(task)->state;
+	ret = (state & CGROUP_FREEZING) && !(state & CGROUP_FROZEN);
 	rcu_read_unlock();
 
 	return ret;
-- 
2.43.0.rc0.421.g78406f8d94-goog