From: Shakeel Butt <shakeelb@google.com>

Date: Sun, 24 Oct 2021 23:19:14 -0700

Subject: cgroup: reduce dependency on cgroup_mutex

Git-commit: be288169712f3dea0bc6b50c00b3ab53d85f1435

Patch-mainline: v5.16-rc1

References: bsc#1205650

Currently cgroup_get_from_path() and cgroup_get_from_id() grab

cgroup_mutex before traversing the default hierarchy to find the

kernfs_node corresponding to the path/id and then extract the linked

cgroup. Since cgroup_mutex is still held, it is guaranteed that the

cgroup will be alive and the reference can be taken on it.

However similar guarantee can be provided without depending on the

cgroup_mutex and potentially reducing avenues of cgroup_mutex contentions.

The kernfs_node's priv pointer is RCU protected pointer and with just

rcu read lock we can grab the reference on the cgroup without

cgroup_mutex. So, remove cgroup_mutex from them.

Signed-off-by: Shakeel Butt <shakeelb@google.com>

Signed-off-by: Tejun Heo <tj@kernel.org>

Acked-by: Michal Koutný <mkoutny@suse.com>

---

 kernel/cgroup/cgroup.c | 51 ++++++++++++++++++++++++++++----------------------

 1 file changed, 29 insertions(+), 22 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c

index 2e98db4558f2..003204c85893 100644

--- a/kernel/cgroup/cgroup.c

+++ b/kernel/cgroup/cgroup.c

@@ -5932,17 +5932,20 @@ struct cgroup *cgroup_get_from_id(u64 id)

 	struct kernfs_node *kn;

 	struct cgroup *cgrp = NULL;

-	mutex_lock(&cgroup_mutex);

 	kn = kernfs_find_and_get_node_by_id(cgrp_dfl_root.kf_root, id);

 	if (!kn)

-		goto out_unlock;

+		goto out;

+

+	rcu_read_lock();

-	cgrp = kn->priv;

-	if (cgroup_is_dead(cgrp) || !cgroup_tryget(cgrp))

+	cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv);

+	if (cgrp && !cgroup_tryget(cgrp))

 		cgrp = NULL;

+

+	rcu_read_unlock();

+

 	kernfs_put(kn);

-out_unlock:

-	mutex_unlock(&cgroup_mutex);

+out:

 	return cgrp;

 }

 EXPORT_SYMBOL_GPL(cgroup_get_from_id);

@@ -6495,30 +6498,34 @@ struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss)

  *

  * Find the cgroup at @path on the default hierarchy, increment its

  * reference count and return it.  Returns pointer to the found cgroup on

- * success, ERR_PTR(-ENOENT) if @path doesn't exist and ERR_PTR(-ENOTDIR)

- * if @path points to a non-directory.

+ * success, ERR_PTR(-ENOENT) if @path doesn't exist or if the cgroup has already

+ * been released and ERR_PTR(-ENOTDIR) if @path points to a non-directory.

  */

 struct cgroup *cgroup_get_from_path(const char *path)

 {

 	struct kernfs_node *kn;

-	struct cgroup *cgrp;

-

-	mutex_lock(&cgroup_mutex);

+	struct cgroup *cgrp = ERR_PTR(-ENOENT);

 	kn = kernfs_walk_and_get(cgrp_dfl_root.cgrp.kn, path);

-	if (kn) {

-		if (kernfs_type(kn) == KERNFS_DIR) {

-			cgrp = kn->priv;

-			cgroup_get_live(cgrp);

-		} else {

-			cgrp = ERR_PTR(-ENOTDIR);

-		}

-		kernfs_put(kn);

-	} else {

-		cgrp = ERR_PTR(-ENOENT);

+	if (!kn)

+		goto out;

+

+	if (kernfs_type(kn) != KERNFS_DIR) {

+		cgrp = ERR_PTR(-ENOTDIR);

+		goto out_kernfs;

 	}

-	mutex_unlock(&cgroup_mutex);

+	rcu_read_lock();

+

+	cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv);

+	if (!cgrp || !cgroup_tryget(cgrp))

+		cgrp = ERR_PTR(-ENOENT);

+

+	rcu_read_unlock();

+

+out_kernfs:

+	kernfs_put(kn);

+out:

 	return cgrp;

 }

 EXPORT_SYMBOL_GPL(cgroup_get_from_path);