[Devel] [PATCH RH7 2/4] exit: reparent: document the ->has_child_subreaper checks

[Pavel Tikhomirov]

From: Oleg Nesterov <oleg at redhat.com>

Swap the "init_task" and same_thread_group() checks.  This way it is more
simple to document these checks and we can remove the link to the previous
discussion on lkml.

Signed-off-by: Oleg Nesterov <oleg at redhat.com>
Cc: Aaron Tomlin <atomlin at redhat.com>
Cc: "Eric W. Biederman" <ebiederm at xmission.com>
Cc: Kay Sievers <kay at vrfy.org>
Cc: Lennart Poettering <lennart at poettering.net>
Cc: Sterling Alexander <stalexan at redhat.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

(cherry picked from commit 175aed3f8d38b87d3287bb765c794205f2b511de)
Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
---
 kernel/exit.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index ff6c58f12d8f..b87564b77011 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -643,18 +643,16 @@ static struct task_struct *find_new_reaper(struct task_struct *father)
 
 	if (father->signal->has_child_subreaper) {
 		struct task_struct *reaper;
-
 		/*
-		 * Find the first ancestor marked as child_subreaper.
-		 * Note that the code below checks same_thread_group(reaper,
-		 * pid_ns->child_reaper).  This is what we need to DTRT in a
-		 * PID namespace. However we still need the check above, see
-		 * http://marc.info/?l=linux-kernel&m=131385460420380
+		 * Find the first ->is_child_subreaper ancestor in our pid_ns.
+		 * We start from father to ensure we can not look into another
+		 * namespace, this is safe because all its threads are dead.
 		 */
 		for (reaper = father;
-		     reaper != &init_task;
+		     !same_thread_group(reaper, pid_ns->child_reaper);
 		     reaper = reaper->real_parent) {
-			if (same_thread_group(reaper, pid_ns->child_reaper))
+			/* call_usermodehelper() descendants need this check */
+			if (reaper == &init_task)
 				break;
 			if (!reaper->signal->is_child_subreaper)
 				continue;
-- 
2.35.3