1 files changed, 27 insertions, 35 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index b409df3b2e9d..d71e012e81be 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1762,7 +1762,7 @@ EXPORT_SYMBOL_GPL(cgroup_path);
  *
  * 'guarantee' is set if the caller promises that a new css_set for the task
  * will already exist. If not set, this function might sleep, and can fail with
- * -ENOMEM. Otherwise, it can only fail with -ESRCH.
+ * -ENOMEM. Must be called with cgroup_mutex and threadgroup locked.
  */
 static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
 			       struct task_struct *tsk, bool guarantee)
@@ -1800,13 +1800,9 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
 	}
 	put_css_set(oldcg);
 
-	/* if PF_EXITING is set, the tsk->cgroups pointer is no longer safe. */
+	/* @tsk can't exit as its threadgroup is locked */
 	task_lock(tsk);
-	if (tsk->flags & PF_EXITING) {
-		task_unlock(tsk);
-		put_css_set(newcg);
-		return -ESRCH;
-	}
+	WARN_ON_ONCE(tsk->flags & PF_EXITING);
 	rcu_assign_pointer(tsk->cgroups, newcg);
 	task_unlock(tsk);
 
@@ -1832,8 +1828,8 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
  * @cgrp: the cgroup the task is attaching to
  * @tsk: the task to be attached
  *
- * Call holding cgroup_mutex. May take task_lock of
- * the task 'tsk' during call.
+ * Call with cgroup_mutex and threadgroup locked. May take task_lock of
+ * @tsk during call.
  */
 int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 {
@@ -1842,6 +1838,10 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 	struct cgroup *oldcgrp;
 	struct cgroupfs_root *root = cgrp->root;
 
+	/* @tsk either already exited or can't exit until the end */
+	if (tsk->flags & PF_EXITING)
+		return -ESRCH;
+
 	/* Nothing to do if the task is already in that cgroup */
 	oldcgrp = task_cgroup_from_root(tsk, root);
 	if (cgrp == oldcgrp)
@@ -2062,6 +2062,10 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 	tsk = leader;
 	i = 0;
 	do {
+		/* @tsk either already exited or can't exit until the end */
+		if (tsk->flags & PF_EXITING)
+			continue;
+
 		/* as per above, nr_threads may decrease, but not increase. */
 		BUG_ON(i >= group_size);
 		get_task_struct(tsk);
@@ -2116,11 +2120,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 			continue;
 		/* get old css_set pointer */
 		task_lock(tsk);
-		if (tsk->flags & PF_EXITING) {
-			/* ignore this task if it's going away */
-			task_unlock(tsk);
-			continue;
-		}
 		oldcg = tsk->cgroups;
 		get_css_set(oldcg);
 		task_unlock(tsk);
@@ -2153,16 +2152,12 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 		oldcgrp = task_cgroup_from_root(tsk, root);
 		if (cgrp == oldcgrp)
 			continue;
-		/* if the thread is PF_EXITING, it can just get skipped. */
 		retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, true);
-		if (retval == 0) {
-			/* attach each task to each subsystem */
-			for_each_subsys(root, ss) {
-				if (ss->attach_task)
-					ss->attach_task(cgrp, tsk);
-			}
-		} else {
-			BUG_ON(retval != -ESRCH);
+		BUG_ON(retval);
+		/* attach each task to each subsystem */
+		for_each_subsys(root, ss) {
+			if (ss->attach_task)
+				ss->attach_task(cgrp, tsk);
 		}
 	}
 	/* nothing is sensitive to fork() after this point. */
@@ -2215,8 +2210,8 @@ out_free_group_list:
 
 /*
  * Find the task_struct of the task to attach by vpid and pass it along to the
- * function to attach either it or all tasks in its threadgroup. Will take
- * cgroup_mutex; may take task_lock of task.
+ * function to attach either it or all tasks in its threadgroup. Will lock
+ * cgroup_mutex and threadgroup; may take task_lock of task.
  */
 static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
 {
@@ -2243,11 +2238,6 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
 			 * detect it later.
 			 */
 			tsk = tsk->group_leader;
-		} else if (tsk->flags & PF_EXITING) {
-			/* optimization for the single-task-only case */
-			rcu_read_unlock();
-			cgroup_unlock();
-			return -ESRCH;
 		}
 		/*
 		 * even if we're attaching all tasks in the thread group, we
@@ -2271,13 +2261,15 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
 		get_task_struct(tsk);
 	}
 
-	if (threadgroup) {
-		threadgroup_lock(tsk);
+	threadgroup_lock(tsk);
+
+	if (threadgroup)
 		ret = cgroup_attach_proc(cgrp, tsk);
-		threadgroup_unlock(tsk);
-	} else {
+	else
 		ret = cgroup_attach_task(cgrp, tsk);
-	}
+
+	threadgroup_unlock(tsk);
+
 	put_task_struct(tsk);
 	cgroup_unlock();
 	return ret;