cgroup: update locking in cgroup_show_options()

cgroup_show_options() grabs cgroup_root_mutex to protect the options
changing while printing; however, holding root_mutex or not doesn't
really make much difference for the function.  subsys_mask can be
atomically tested and most of the options aren't allowed to change
anyway once mounted.

The only field which needs synchronization is ->release_agent_path.
This patch introduces a dedicated spinlock to synchronize accesses to
the field and drops cgroup_root_mutex locking from
cgroup_show_options().  The next patch will remove cgroup_root_mutex.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

kernel/cgroup.c

@@ -92,6 +92,12 @@ static DEFINE_MUTEX(cgroup_mutex);
 
 static DEFINE_MUTEX(cgroup_root_mutex);
 
+/*
+ * Protects cgroup_subsys->release_agent_path.  Modifying it also requires
+ * cgroup_mutex.  Reading requires either cgroup_mutex or this spinlock.
+ */
+static DEFINE_SPINLOCK(release_agent_path_lock);
+
 #define cgroup_assert_mutex_or_rcu_locked()				\
 	rcu_lockdep_assert(rcu_read_lock_held() ||			\
 			   lockdep_is_held(&cgroup_mutex),		\
@@ -1034,7 +1040,6 @@ static int cgroup_show_options(struct seq_file *seq, struct dentry *dentry)
 	struct cgroup_subsys *ss;
 	int ssid;
 
-	mutex_lock(&cgroup_root_mutex);
 	for_each_subsys(ss, ssid)
 		if (root->subsys_mask & (1 << ssid))
 			seq_printf(seq, ",%s", ss->name);
@@ -1044,13 +1049,16 @@ static int cgroup_show_options(struct seq_file *seq, struct dentry *dentry)
 		seq_puts(seq, ",noprefix");
 	if (root->flags & CGRP_ROOT_XATTR)
 		seq_puts(seq, ",xattr");
+
+	spin_lock(&release_agent_path_lock);
 	if (strlen(root->release_agent_path))
 		seq_printf(seq, ",release_agent=%s", root->release_agent_path);
+	spin_unlock(&release_agent_path_lock);
+
 	if (test_bit(CGRP_CPUSET_CLONE_CHILDREN, &root->top_cgroup.flags))
 		seq_puts(seq, ",clone_children");
 	if (strlen(root->name))
 		seq_printf(seq, ",name=%s", root->name);
-	mutex_unlock(&cgroup_root_mutex);
 	return 0;
 }
 
@@ -1272,8 +1280,11 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
 	if (ret)
 		goto out_unlock;
 
-	if (opts.release_agent)
+	if (opts.release_agent) {
+		spin_lock(&release_agent_path_lock);
 		strcpy(root->release_agent_path, opts.release_agent);
+		spin_unlock(&release_agent_path_lock);
+	}
  out_unlock:
 	kfree(opts.release_agent);
 	kfree(opts.name);
@@ -2183,7 +2194,9 @@ static int cgroup_release_agent_write(struct cgroup_subsys_state *css,
 	if (!cgroup_lock_live_group(css->cgroup))
 		return -ENODEV;
 	mutex_lock(&cgroup_root_mutex);
+	spin_lock(&release_agent_path_lock);
 	strcpy(css->cgroup->root->release_agent_path, buffer);
+	spin_unlock(&release_agent_path_lock);
 	mutex_unlock(&cgroup_root_mutex);
 	mutex_unlock(&cgroup_mutex);
 	return 0;