1 files changed, 43 insertions, 38 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index be81342caf1b..7430852a8571 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -438,14 +438,46 @@ static void coredump_task_exit(struct task_struct *tsk)
 }
 
 #ifdef CONFIG_MEMCG
+/* drops tasklist_lock if succeeds */
+static bool __try_to_set_owner(struct task_struct *tsk, struct mm_struct *mm)
+{
+	bool ret = false;
+
+	task_lock(tsk);
+	if (likely(tsk->mm == mm)) {
+		/* tsk can't pass exit_mm/exec_mmap and exit */
+		read_unlock(&tasklist_lock);
+		WRITE_ONCE(mm->owner, tsk);
+		lru_gen_migrate_mm(mm);
+		ret = true;
+	}
+	task_unlock(tsk);
+	return ret;
+}
+
+static bool try_to_set_owner(struct task_struct *g, struct mm_struct *mm)
+{
+	struct task_struct *t;
+
+	for_each_thread(g, t) {
+		struct mm_struct *t_mm = READ_ONCE(t->mm);
+		if (t_mm == mm) {
+			if (__try_to_set_owner(t, mm))
+				return true;
+		} else if (t_mm)
+			break;
+	}
+
+	return false;
+}
+
 /*
  * A task is exiting.   If it owned this mm, find a new owner for the mm.
  */
 void mm_update_next_owner(struct mm_struct *mm)
 {
-	struct task_struct *c, *g, *p = current;
+	struct task_struct *g, *p = current;
 
-retry:
 	/*
 	 * If the exiting or execing task is not the owner, it's
 	 * someone else's problem.
@@ -466,19 +498,17 @@ retry:
 	/*
 	 * Search in the children
 	 */
-	list_for_each_entry(c, &p->children, sibling) {
-		if (c->mm == mm)
-			goto assign_new_owner;
+	list_for_each_entry(g, &p->children, sibling) {
+		if (try_to_set_owner(g, mm))
+			goto ret;
 	}
-
 	/*
 	 * Search in the siblings
 	 */
-	list_for_each_entry(c, &p->real_parent->children, sibling) {
-		if (c->mm == mm)
-			goto assign_new_owner;
+	list_for_each_entry(g, &p->real_parent->children, sibling) {
+		if (try_to_set_owner(g, mm))
+			goto ret;
 	}
-
 	/*
 	 * Search through everything else, we should not get here often.
 	 */
@@ -487,12 +517,8 @@ retry:
 			break;
 		if (g->flags & PF_KTHREAD)
 			continue;
-		for_each_thread(g, c) {
-			if (c->mm == mm)
-				goto assign_new_owner;
-			if (c->mm)
-				break;
-		}
+		if (try_to_set_owner(g, mm))
+			goto ret;
 	}
 	read_unlock(&tasklist_lock);
 	/*
@@ -501,30 +527,9 @@ retry:
 	 * ptrace or page migration (get_task_mm()).  Mark owner as NULL.
 	 */
 	WRITE_ONCE(mm->owner, NULL);
+ ret:
 	return;
 
-assign_new_owner:
-	BUG_ON(c == p);
-	get_task_struct(c);
-	/*
-	 * The task_lock protects c->mm from changing.
-	 * We always want mm->owner->mm == mm
-	 */
-	task_lock(c);
-	/*
-	 * Delay read_unlock() till we have the task_lock()
-	 * to ensure that c does not slip away underneath us
-	 */
-	read_unlock(&tasklist_lock);
-	if (c->mm != mm) {
-		task_unlock(c);
-		put_task_struct(c);
-		goto retry;
-	}
-	WRITE_ONCE(mm->owner, c);
-	lru_gen_migrate_mm(mm);
-	task_unlock(c);
-	put_task_struct(c);
 }
 #endif /* CONFIG_MEMCG */