exit: reparent: document the ->has_child_subreaper checks

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

1 files changed, 6 insertions, 8 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index a4204aaba8a2..576949ce5665 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -495,18 +495,16 @@ 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;