exit: reparent: call forget_original_parent() under tasklist_lock

Shift "release dead children" loop from forget_original_parent() to its
caller, exit_notify().  It is safe to reap them even if our parent reaps
us right after we drop tasklist_lock, those children no longer have any
connection to the exiting task.

And this allows us to avoid write_lock_irq(tasklist_lock) right after it
was released by forget_original_parent(), we can simply call it with
tasklist_lock held.

While at it, move the comment about forget_original_parent() up to
this function.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Aaron Tomlin <atomlin@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
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, 23 insertions, 24 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 063745699f7f..8061891ddd9b 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -560,19 +560,26 @@ static void reparent_leader(struct task_struct *father, struct task_struct *p,
         kill_orphaned_pgrp(p, father);
 }
 
-static void forget_original_parent(struct task_struct *father)
+/*
+ * This does two things:
+ *
+ * A.  Make init inherit all the child processes
+ * B.  Check to see if any process groups have become orphaned
+ *      as a result of our exiting, and if they have any stopped
+ *      jobs, send them a SIGHUP and then a SIGCONT.  (POSIX 3.2.2.2)
+ */
+static void forget_original_parent(struct task_struct *father,
+                                        struct list_head *dead)
 {
-        struct task_struct *p, *t, *n, *reaper;
-        LIST_HEAD(dead_children);
+        struct task_struct *p, *t, *reaper;
 
-        write_lock_irq(&tasklist_lock);
         if (unlikely(!list_empty(&father->ptraced)))
-                exit_ptrace(father, &dead_children);
+                exit_ptrace(father, dead);
 
         /* Can drop and reacquire tasklist_lock */
         reaper = find_child_reaper(father);
         if (list_empty(&father->children))
-                goto unlock;
+                return;
 
         reaper = find_new_reaper(father, reaper);
         list_for_each_entry(p, &father->children, sibling) {
@@ -590,16 +597,9 @@ static void forget_original_parent(struct task_struct *father)
                  * notify anyone anything has happened.
                  */
                 if (!same_thread_group(reaper, father))
-                        reparent_leader(father, p, &dead_children);
+                        reparent_leader(father, p, dead);
         }
         list_splice_tail_init(&father->children, &reaper->children);
- unlock:
-        write_unlock_irq(&tasklist_lock);
-
-        list_for_each_entry_safe(p, n, &dead_children, ptrace_entry) {
-                list_del_init(&p->ptrace_entry);
-                release_task(p);
-        }
 }
 
 /*
@@ -609,18 +609,12 @@ static void forget_original_parent(struct task_struct *father)
 static void exit_notify(struct task_struct *tsk, int group_dead)
 {
         bool autoreap;
-
-        /*
-         * This does two things:
-         *
-         * A.  Make init inherit all the child processes
-         * B.  Check to see if any process groups have become orphaned
-         *      as a result of our exiting, and if they have any stopped
-         *      jobs, send them a SIGHUP and then a SIGCONT.  (POSIX 3.2.2.2)
-         */
-        forget_original_parent(tsk);
+        struct task_struct *p, *n;
+        LIST_HEAD(dead);
 
         write_lock_irq(&tasklist_lock);
+        forget_original_parent(tsk, &dead);
+
         if (group_dead)
                 kill_orphaned_pgrp(tsk->group_leader, NULL);
 
@@ -644,6 +638,11 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
                 wake_up_process(tsk->signal->group_exit_task);
         write_unlock_irq(&tasklist_lock);
 
+        list_for_each_entry_safe(p, n, &dead, ptrace_entry) {
+                list_del_init(&p->ptrace_entry);
+                release_task(p);
+        }
+
         /* If the process is dead, release it - nobody will wait for it */
         if (autoreap)
                 release_task(tsk);