Report detached thread exit to the debugger

diff -Nru a/kernel/exit.c b/kernel/exit.c
@@ -651,6 +651,8 @@
 	if (tsk->exit_signal != -1) {
 		int signal = tsk->parent == tsk->real_parent ? tsk->exit_signal : SIGCHLD;
 		do_notify_parent(tsk, signal);
+	} else if (tsk->ptrace) {
+		do_notify_parent(tsk, SIGCHLD);
 	}
 
 	tsk->state = TASK_ZOMBIE;

@@ -715,7 +717,7 @@
 	tsk->exit_code = code;
 	exit_notify(tsk);
 
-	if (tsk->exit_signal == -1)
+	if (tsk->exit_signal == -1 && tsk->ptrace == 0)
 		release_task(tsk);
 
 	schedule();

@@ -859,7 +861,7 @@
 		BUG_ON(state != TASK_DEAD);
 		return 0;
 	}
-	if (unlikely(p->exit_signal == -1))
+	if (unlikely(p->exit_signal == -1 && p->ptrace == 0))
 		/*
 		 * This can only happen in a race with a ptraced thread
 		 * dying on another processor.

@@ -889,8 +891,12 @@
 		/* Double-check with lock held.  */
 		if (p->real_parent != p->parent) {
 			__ptrace_unlink(p);
-			do_notify_parent(p, p->exit_signal);
 			p->state = TASK_ZOMBIE;
+			/* If this is a detached thread, this is where it goes away.  */
+			if (p->exit_signal == -1)
+				release_task (p);
+			else
+				do_notify_parent(p, p->exit_signal);
 			p = NULL;
 		}
 		write_unlock_irq(&tasklist_lock);