exit: reparent: cleanup the usage of reparent_leader()

1. Now that reparent_leader() doesn't abuse ->sibling we can shift
   list_move_tail() from reparent_leader() to forget_original_parent()
   and turn it into a single list_splice_tail_init(). This also makes
   BUG_ON(!list_empty()) and list_for_each_entry_safe() unnecessary.

2. This also allows to shift the same_thread_group() check, it looks
   a bit more clear in the caller.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Aaron Tomlin <atomlin@redhat.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
Cc: Sterling Alexander <stalexan@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Roland McGrath <roland@hack.frob.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 13 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 464971e6923e..772e9175735c 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -529,15 +529,7 @@ static struct task_struct *find_new_reaper(struct task_struct *father)
 static void reparent_leader(struct task_struct *father, struct task_struct *p,
                                 struct list_head *dead)
 {
-        list_move_tail(&p->sibling, &p->real_parent->children);
-
-        if (p->exit_state == EXIT_DEAD)
-                return;
-        /*
-         * If this is a threaded reparent there is no need to
-         * notify anyone anything has happened.
-         */
-        if (same_thread_group(p->real_parent, father))
+        if (unlikely(p->exit_state == EXIT_DEAD))
                 return;
 
         /* We don't want people slaying init. */
@@ -568,7 +560,7 @@ static void forget_original_parent(struct task_struct *father)
         exit_ptrace(father);
         reaper = find_new_reaper(father);
 
-        list_for_each_entry_safe(p, n, &father->children, sibling) {
+        list_for_each_entry(p, &father->children, sibling) {
                 for_each_thread(p, t) {
                         t->real_parent = reaper;
                         BUG_ON((!t->ptrace) != (t->parent == father));
@@ -578,12 +570,16 @@ static void forget_original_parent(struct task_struct *father)
                                 group_send_sig_info(t->pdeath_signal,
                                                     SEND_SIG_NOINFO, t);
                 }
-                reparent_leader(father, p, &dead_children);
+                /*
+                 * If this is a threaded reparent there is no need to
+                 * notify anyone anything has happened.
+                 */
+                if (!same_thread_group(reaper, father))
+                        reparent_leader(father, p, &dead_children);
         }
+        list_splice_tail_init(&father->children, &reaper->children);
         write_unlock_irq(&tasklist_lock);
 
-        BUG_ON(!list_empty(&father->children));
-
         list_for_each_entry_safe(p, n, &dead_children, ptrace_entry) {
                 list_del_init(&p->ptrace_entry);
                 release_task(p);