device_cgroup: simplify cgroup tree walk in propagate_exception()

During a config change, propagate_exception() needs to traverse the
subtree to update config on the subtree.  Because such config updates
need to allocate memory, it couldn't directly use
cgroup_for_each_descendant_pre() which required the whole iteration to
be contained in a single RCU read critical section.  To work around
the limitation, propagate_exception() built a linked list of
descendant cgroups while read-locking RCU and then walked the list
afterwards, which is safe as the whole iteration is protected by
devcgroup_mutex.  This works but is cumbersome.

With the recent updates, cgroup iterators now allow dropping RCU read
lock while iteration is in progress making this workaround no longer
necessary.  This patch replaces dev_cgroup->propagate_pending list and
get_online_devcg() with direct cgroup_for_each_descendant_pre() walk.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Aristeu Rozanski <aris@redhat.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>

security/device_cgroup.c

@@ -49,8 +49,6 @@ struct dev_cgroup {
 	struct cgroup_subsys_state css;
 	struct list_head exceptions;
 	enum devcg_behavior behavior;
-	/* temporary list for pending propagation operations */
-	struct list_head propagate_pending;
 };
 
 static inline struct dev_cgroup *css_to_devcgroup(struct cgroup_subsys_state *s)
@@ -241,7 +239,6 @@ static struct cgroup_subsys_state *devcgroup_css_alloc(struct cgroup *cgroup)
 	if (!dev_cgroup)
 		return ERR_PTR(-ENOMEM);
 	INIT_LIST_HEAD(&dev_cgroup->exceptions);
-	INIT_LIST_HEAD(&dev_cgroup->propagate_pending);
 	dev_cgroup->behavior = DEVCG_DEFAULT_NONE;
 
 	return &dev_cgroup->css;
@@ -444,34 +441,6 @@ static void revalidate_active_exceptions(struct dev_cgroup *devcg)
 	}
 }
 
-/**
- * get_online_devcg - walks the cgroup tree and fills a list with the online
- * 		      groups
- * @root: cgroup used as starting point
- * @online: list that will be filled with online groups
- *
- * Must be called with devcgroup_mutex held. Grabs RCU lock.
- * Because devcgroup_mutex is held, no devcg will become online or offline
- * during the tree walk (see devcgroup_online, devcgroup_offline)
- * A separated list is needed because propagate_behavior() and
- * propagate_exception() need to allocate memory and can block.
- */
-static void get_online_devcg(struct cgroup *root, struct list_head *online)
-{
-	struct cgroup *pos;
-	struct dev_cgroup *devcg;
-
-	lockdep_assert_held(&devcgroup_mutex);
-
-	rcu_read_lock();
-	cgroup_for_each_descendant_pre(pos, root) {
-		devcg = cgroup_to_devcgroup(pos);
-		if (is_devcg_online(devcg))
-			list_add_tail(&devcg->propagate_pending, online);
-	}
-	rcu_read_unlock();
-}
-
 /**
  * propagate_exception - propagates a new exception to the children
  * @devcg_root: device cgroup that added a new exception
@@ -482,15 +451,24 @@ static void get_online_devcg(struct cgroup *root, struct list_head *online)
 static int propagate_exception(struct dev_cgroup *devcg_root,
 			       struct dev_exception_item *ex)
 {
-	struct cgroup *root = devcg_root->css.cgroup;
-	struct dev_cgroup *devcg, *parent, *tmp;
+	struct cgroup *root = devcg_root->css.cgroup, *pos;
 	int rc = 0;
-	LIST_HEAD(pending);
 
-	get_online_devcg(root, &pending);
+	rcu_read_lock();
 
-	list_for_each_entry_safe(devcg, tmp, &pending, propagate_pending) {
-		parent = cgroup_to_devcgroup(devcg->css.cgroup->parent);
+	cgroup_for_each_descendant_pre(pos, root) {
+		struct dev_cgroup *devcg = cgroup_to_devcgroup(pos);
+
+		/*
+		 * Because devcgroup_mutex is held, no devcg will become
+		 * online or offline during the tree walk (see on/offline
+		 * methods), and online ones are safe to access outside RCU
+		 * read lock without bumping refcnt.
+		 */
+		if (!is_devcg_online(devcg))
+			continue;
+
+		rcu_read_unlock();
 
 		/*
 		 * in case both root's behavior and devcg is allow, a new
@@ -512,8 +490,10 @@ static int propagate_exception(struct dev_cgroup *devcg_root,
 		}
 		revalidate_active_exceptions(devcg);
 
-		list_del_init(&devcg->propagate_pending);
+		rcu_read_lock();
 	}
+
+	rcu_read_unlock();
 	return rc;
 }