[PATCH 2/3] memcg: change assign_new_owner() to consider the sub-htreads

[Oleg Nesterov <oleg@redhat.com>]

mm_update_next_owner() checks the children and siblings first but
it only inspects the group leaders, and thus this optimization won't
work if the leader is zombie.

This is actually correct, the last for_each_process() loop will find
these children/siblings again, but this doesn't look consistent/clean.

Move the for_each_thread() logic from mm_update_next_owner() to
assign_new_owner(). We can also remove the "struct task_struct *c"
local.

See also the next patch relies on this change.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/exit.c |   39 ++++++++++++++++++++-------------------
 1 files changed, 20 insertions(+), 19 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 4d446ab..1d1810d 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -293,19 +293,24 @@ kill_orphaned_pgrp(struct task_struct *tsk, struct task_struct *parent)
 }
 
 #ifdef CONFIG_MEMCG
-static bool assign_new_owner(struct mm_struct *mm, struct task_struct *c)
+static bool assign_new_owner(struct mm_struct *mm, struct task_struct *g)
 {
+	struct task_struct *c;
 	bool ret = false;
 
-	if (c->mm != mm)
-		return ret;
+	for_each_thread(g, c) {
+		if (c->mm == mm) {
+			task_lock(c); /* protects c->mm from changing */
+			if (c->mm == mm) {
+				mm->owner = c;
+				ret = true;
+			}
+			task_unlock(c);
+		}
 
-	task_lock(c); /* protects c->mm from changing */
-	if (c->mm == mm) {
-		mm->owner = c;
-		ret = true;
+		if (ret || c->mm)
+			break;
 	}
-	task_unlock(c);
 
 	return ret;
 }
@@ -315,7 +320,7 @@ static bool assign_new_owner(struct mm_struct *mm, struct task_struct *c)
  */
 void mm_update_next_owner(struct mm_struct *mm)
 {
-	struct task_struct *c, *g, *p = current;
+	struct task_struct *g, *p = current;
 
 	/*
 	 * If the exiting or execing task is not the owner, it's
@@ -337,16 +342,16 @@ void mm_update_next_owner(struct mm_struct *mm)
 	/*
 	 * Search in the children
 	 */
-	list_for_each_entry(c, &p->children, sibling) {
-		if (assign_new_owner(mm, c))
+	list_for_each_entry(g, &p->children, sibling) {
+		if (assign_new_owner(mm, g))
 			goto done;
 	}
 
 	/*
 	 * Search in the siblings
 	 */
-	list_for_each_entry(c, &p->real_parent->children, sibling) {
-		if (assign_new_owner(mm, c))
+	list_for_each_entry(g, &p->real_parent->children, sibling) {
+		if (assign_new_owner(mm, g))
 			goto done;
 	}
 
@@ -356,12 +361,8 @@ void mm_update_next_owner(struct mm_struct *mm)
 	for_each_process(g) {
 		if (g->flags & PF_KTHREAD)
 			continue;
-		for_each_thread(g, c) {
-			if (assign_new_owner(mm, c))
-				goto done;
-			if (c->mm)
-				break;
-		}
+		if (assign_new_owner(mm, g))
+			goto done;
 	}
 
 	/*
-- 
1.5.5.1


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