ptrace: simplify ptrace_exit()->ignoring_children() path

ignoring_children() takes parent->sighand->siglock and checks
k_sigaction[SIGCHLD] atomically.  But this buys nothing, we can't get the
"really" wrong result even if we race with sigaction(SIGCHLD).  If we read
the "stale" sa_handler/sa_flags we can pretend it was changed right after
the check.

Remove spin_lock(->siglock), and kill "int ign" which caches the result of
ignoring_children() which becomes rather trivial.

Perhaps it makes sense to export this helper, do_notify_parent() can use
it too.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Jerome Marchand <jmarchan@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -732,19 +732,15 @@ static void exit_mm(struct task_struct * tsk)
 }
 
 /*
- * Return nonzero if @parent's children should reap themselves.
- *
- * Called with write_lock_irq(&tasklist_lock) held.
+ * Called with irqs disabled, returns true if childs should reap themselves.
  */
-static int ignoring_children(struct task_struct *parent)
+static int ignoring_children(struct sighand_struct *sigh)
 {
 	int ret;
-	struct sighand_struct *psig = parent->sighand;
-	unsigned long flags;
-	spin_lock_irqsave(&psig->siglock, flags);
-	ret = (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN ||
-	       (psig->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT));
-	spin_unlock_irqrestore(&psig->siglock, flags);
+	spin_lock(&sigh->siglock);
+	ret = (sigh->action[SIGCHLD-1].sa.sa_handler == SIG_IGN) ||
+	      (sigh->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT);
+	spin_unlock(&sigh->siglock);
 	return ret;
 }
 
@@ -757,7 +753,6 @@ static int ignoring_children(struct task_struct *parent)
 static void ptrace_exit(struct task_struct *parent, struct list_head *dead)
 {
 	struct task_struct *p, *n;
-	int ign = -1;
 
 	list_for_each_entry_safe(p, n, &parent->ptraced, ptrace_entry) {
 		__ptrace_unlink(p);
@@ -779,12 +774,8 @@ static void ptrace_exit(struct task_struct *parent, struct list_head *dead)
 		if (!task_detached(p) && thread_group_empty(p)) {
 			if (!same_thread_group(p->real_parent, parent))
 				do_notify_parent(p, p->exit_signal);
-			else {
-				if (ign < 0)
-					ign = ignoring_children(parent);
-				if (ign)
-					p->exit_signal = -1;
-			}
+			else if (ignoring_children(parent->sighand))
+				p->exit_signal = -1;
 		}
 
 		if (task_detached(p)) {