cgroup: hold cgroup_mutex before calling css_offline()

cpuset no longer nests cgroup_mutex inside cpu_hotplug lock, so
we don't have to release cgroup_mutex before calling css_offline().
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Documentation/cgroups/cgroups.txt

@@ -580,6 +580,7 @@ propagation along the hierarchy. See the comment on
 cgroup_for_each_descendant_pre() for details.
 
 void css_offline(struct cgroup *cgrp);
+(cgroup_mutex held by caller)
 
 This is the counterpart of css_online() and called iff css_online()
 has succeeded on @cgrp. This signifies the beginning of the end of

kernel/cgroup.c

@@ -4169,17 +4169,8 @@ static void offline_css(struct cgroup_subsys *ss, struct cgroup *cgrp)
 	if (!(css->flags & CSS_ONLINE))
 		return;
 
-	/*
-	 * css_offline() should be called with cgroup_mutex unlocked.  See
-	 * 3fa59dfbc3 ("cgroup: fix potential deadlock in pre_destroy") for
-	 * details.  This temporary unlocking should go away once
-	 * cgroup_mutex is unexported from controllers.
-	 */
-	if (ss->css_offline) {
-		mutex_unlock(&cgroup_mutex);
+	if (ss->css_offline)
 		ss->css_offline(cgrp);
-		mutex_lock(&cgroup_mutex);
-	}
 
 	cgrp->subsys[ss->subsys_id]->flags &= ~CSS_ONLINE;
 }