Patch "cgroup: reduce dependency on cgroup_mutex" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    cgroup: reduce dependency on cgroup_mutex

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     cgroup-reduce-dependency-on-cgroup_mutex.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 5545e127d9dd0ed4f21ba449fdb6e55db5aa1af7
Author: Shakeel Butt <shakeelb@xxxxxxxxxx>
Date:   Sun Oct 24 23:19:14 2021 -0700

    cgroup: reduce dependency on cgroup_mutex
    
    [ Upstream commit be288169712f3dea0bc6b50c00b3ab53d85f1435 ]
    
    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@xxxxxxxxxx>
    Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
    Stable-dep-of: df02452f3df0 ("cgroup: cgroup_get_from_id() must check the looked-up kn is a directory")
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 75c3881af078..97282d6b5d18 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -6021,17 +6021,20 @@ 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);
@@ -6585,30 +6588,34 @@ 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);