cgroup: relocate cgroup_attach_lock within cgroup_procs_write_start

Later patches will introduce a new parameter `task` to
cgroup_attach_lock, thus adjusting the position of cgroup_attach_lock
within cgroup_procs_write_start.

Between obtaining the threadgroup leader via PID and acquiring the
cgroup attach lock, the threadgroup leader may change, which could lead
to incorrect cgroup migration. Therefore, after acquiring the cgroup
attach lock, we check whether the threadgroup leader has changed, and if
so, retry the operation.

tj: Minor comment adjustments.

Signed-off-by: Yi Tao <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

kernel/cgroup/cgroup.c

···
 	if (kstrtoint(strstrip(buf), 0, &pid) || pid < 0)
 		return ERR_PTR(-EINVAL);
 
-	/*
-	 * If we migrate a single thread, we don't care about threadgroup
-	 * stability. If the thread is `current`, it won't exit(2) under our
-	 * hands or change PID through exec(2). We exclude
-	 * cgroup_update_dfl_csses and other cgroup_{proc,thread}s_write
-	 * callers by cgroup_mutex.
-	 * Therefore, we can skip the global lock.
-	 */
-	lockdep_assert_held(&cgroup_mutex);
-
-	if (pid || threadgroup)
-		*lock_mode = CGRP_ATTACH_LOCK_GLOBAL;
-	else
-		*lock_mode = CGRP_ATTACH_LOCK_NONE;
-
-	cgroup_attach_lock(*lock_mode);
-
+retry_find_task:
 	rcu_read_lock();
 	if (pid) {
 		tsk = find_task_by_vpid(pid);
 		if (!tsk) {
 			tsk = ERR_PTR(-ESRCH);
-			goto out_unlock_threadgroup;
+			goto out_unlock_rcu;
 		}
 	} else {
 		tsk = current;
···
 	 */
 	if (tsk->no_cgroup_migration || (tsk->flags & PF_NO_SETAFFINITY)) {
 		tsk = ERR_PTR(-EINVAL);
-		goto out_unlock_threadgroup;
+		goto out_unlock_rcu;
 	}
 
 	get_task_struct(tsk);
-	goto out_unlock_rcu;
+	rcu_read_unlock();
 
-out_unlock_threadgroup:
-	cgroup_attach_unlock(*lock_mode);
-	*lock_mode = CGRP_ATTACH_LOCK_NONE;
+	/*
+	 * If we migrate a single thread, we don't care about threadgroup
+	 * stability. If the thread is `current`, it won't exit(2) under our
+	 * hands or change PID through exec(2). We exclude
+	 * cgroup_update_dfl_csses and other cgroup_{proc,thread}s_write callers
+	 * by cgroup_mutex. Therefore, we can skip the global lock.
+	 */
+	lockdep_assert_held(&cgroup_mutex);
+
+	if (pid || threadgroup)
+		*lock_mode = CGRP_ATTACH_LOCK_GLOBAL;
+	else
+		*lock_mode = CGRP_ATTACH_LOCK_NONE;
+
+	cgroup_attach_lock(*lock_mode);
+
+	if (threadgroup) {
+		if (!thread_group_leader(tsk)) {
+			/*
+			 * A race with de_thread from another thread's exec()
+			 * may strip us of our leadership. If this happens,
+			 * throw this task away and try again.
+			 */
+			cgroup_attach_unlock(*lock_mode);
+			put_task_struct(tsk);
+			goto retry_find_task;
+		}
+	}
+
+	return tsk;
+
 out_unlock_rcu:
 	rcu_read_unlock();
 	return tsk;