1 files changed, 23 insertions, 34 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 145deeb69bc3..ad447684aa47 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -386,10 +386,22 @@ static void free_css_set_work(struct work_struct *work)
 		struct cgroup *cgrp = link->cgrp;
 		list_del(&link->cg_link_list);
 		list_del(&link->cgrp_link_list);
-		if (atomic_dec_and_test(&cgrp->count)) {
+
+
+		/*
+		 * We may not be holding cgroup_mutex, and if cgrp->count is
+		 * dropped to 0 the cgroup can be destroyed at any time, hence
+		 * rcu_read_lock is used to keep it alive.
+		 */
+		rcu_read_lock();
+		if (atomic_dec_and_test(&cgrp->count) &&
+		    notify_on_release(cgrp)) {
 			check_for_release(cgrp);
 			cgroup_wakeup_rmdir_waiter(cgrp);
 		}
+
+		rcu_read_unlock();
+
 		kfree(link);
 	}
 	write_unlock(&css_set_lock);
@@ -1878,9 +1890,8 @@ static void cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
 	 * trading it for newcg is protected by cgroup_mutex, we're safe to drop
 	 * it here; it will be freed under RCU.
 	 */
-	put_css_set(oldcg);
-
 	set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
+	put_css_set(oldcg);
 }
 
 /**
@@ -2623,9 +2634,7 @@ static int cgroup_create_dir(struct cgroup *cgrp, struct dentry *dentry,
 		dentry->d_fsdata = cgrp;
 		inc_nlink(parent->d_inode);
 		rcu_assign_pointer(cgrp->dentry, dentry);
-		dget(dentry);
 	}
-	dput(dentry);
 
 	return error;
 }
@@ -4533,31 +4542,20 @@ static const struct file_operations proc_cgroupstats_operations = {
  *
  * A pointer to the shared css_set was automatically copied in
  * fork.c by dup_task_struct().  However, we ignore that copy, since
- * it was not made under the protection of RCU, cgroup_mutex or
- * threadgroup_change_begin(), so it might no longer be a valid
- * cgroup pointer.  cgroup_attach_task() might have already changed
- * current->cgroups, allowing the previously referenced cgroup
- * group to be removed and freed.
- *
- * Outside the pointer validity we also need to process the css_set
- * inheritance between threadgoup_change_begin() and
- * threadgoup_change_end(), this way there is no leak in any process
- * wide migration performed by cgroup_attach_proc() that could otherwise
- * miss a thread because it is too early or too late in the fork stage.
+ * it was not made under the protection of RCU or cgroup_mutex, so
+ * might no longer be a valid cgroup pointer.  cgroup_attach_task() might
+ * have already changed current->cgroups, allowing the previously
+ * referenced cgroup group to be removed and freed.
  *
  * At the point that cgroup_fork() is called, 'current' is the parent
  * task, and the passed argument 'child' points to the child task.
  */
 void cgroup_fork(struct task_struct *child)
 {
-	/*
-	 * We don't need to task_lock() current because current->cgroups
-	 * can't be changed concurrently here. The parent obviously hasn't
-	 * exited and called cgroup_exit(), and we are synchronized against
-	 * cgroup migration through threadgroup_change_begin().
-	 */
+	task_lock(current);
 	child->cgroups = current->cgroups;
 	get_css_set(child->cgroups);
+	task_unlock(current);
 	INIT_LIST_HEAD(&child->cg_list);
 }
 
@@ -4610,19 +4608,10 @@ void cgroup_post_fork(struct task_struct *child)
 	 */
 	if (use_task_css_set_links) {
 		write_lock(&css_set_lock);
-		if (list_empty(&child->cg_list)) {
-			/*
-			 * It's safe to use child->cgroups without task_lock()
-			 * here because we are protected through
-			 * threadgroup_change_begin() against concurrent
-			 * css_set change in cgroup_task_migrate(). Also
-			 * the task can't exit at that point until
-			 * wake_up_new_task() is called, so we are protected
-			 * against cgroup_exit() setting child->cgroup to
-			 * init_css_set.
-			 */
+		task_lock(child);
+		if (list_empty(&child->cg_list))
 			list_add(&child->cg_list, &child->cgroups->tasks);
-		}
+		task_unlock(child);
 		write_unlock(&css_set_lock);
 	}
 }