Merge branch 'for-3.7-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup

Pull cgroup fixes from Tejun Heo:
 "This pull request contains three fixes.

  Two are reverts of task_lock() removal in cgroup fork path.  The
  optimizations incorrectly assumed that threadgroup_lock can protect
  process forks (as opposed to thread creations) too.  Further cleanup
  of cgroup fork path is scheduled.

  The third fixes cgroup emptiness notification loss."

* 'for-3.7-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup:
  Revert "cgroup: Remove task_lock() from cgroup_post_fork()"
  Revert "cgroup: Drop task_lock(parent) on cgroup_fork()"
  cgroup: notify_on_release may not be triggered in some cases

kernel/cgroup.c

@@ -1962,9 +1962,8 @@ 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);
 }
 
 /**
@@ -4815,31 +4814,20 @@ 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);
 }
 
@@ -4895,19 +4883,10 @@ 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);
 	}
 }