linux-kernel - [PATCH 2/3] cgroup: remove cgroup_mutex from cgroupstats

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Message-Id: <20211025061916.3853623-2-shakeelb@google.com>
Date:   Sun, 24 Oct 2021 23:19:15 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     "Michal Koutný" <mkoutny@...e.com>,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Shakeel Butt <shakeelb@...gle.com>
Subject: [PATCH 2/3] cgroup: remove cgroup_mutex from cgroupstats_build

The function cgroupstats_build extracts cgroup from the kernfs_node's
priv pointer which is a RCU pointer. So, there is no need to grab
cgroup_mutex. Just get the reference on the cgroup before using and
remove the cgroup_mutex altogether.

Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
---
 kernel/cgroup/cgroup-v1.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index f6cc5f8484dc..fd14a60379c1 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -698,8 +698,6 @@ int cgroupstats_build(struct cgroupstats *stats, struct dentry *dentry)
 	    kernfs_type(kn) != KERNFS_DIR)
 		return -EINVAL;
 
-	mutex_lock(&cgroup_mutex);
-
 	/*
 	 * We aren't being called from kernfs and there's no guarantee on
 	 * @kn->priv's validity.  For this and css_tryget_online_from_dir(),
@@ -707,9 +705,8 @@ int cgroupstats_build(struct cgroupstats *stats, struct dentry *dentry)
 	 */
 	rcu_read_lock();
 	cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv);
-	if (!cgrp || cgroup_is_dead(cgrp)) {
+	if (!cgrp || !cgroup_tryget(cgrp)) {
 		rcu_read_unlock();
-		mutex_unlock(&cgroup_mutex);
 		return -ENOENT;
 	}
 	rcu_read_unlock();
@@ -737,7 +734,7 @@ int cgroupstats_build(struct cgroupstats *stats, struct dentry *dentry)
 	}
 	css_task_iter_end(&it);
 
-	mutex_unlock(&cgroup_mutex);
+	cgroup_put(cgrp);
 	return 0;
 }
 
-- 
2.33.0.1079.g6e70778dc9-goog