kill tracehook_notify_death()

Kill tracehook_notify_death(), reimplement the logic in its caller,
exit_notify().

Also, change the exec_id's check to use thread_group_leader() instead
of task_detached(), this is more clear. This logic only applies to
the exiting leader, a sub-thread must never change its exit_signal.

Note: when the traced group leader exits the exit_signal-or-SIGCHLD
logic looks really strange:

	- we notify the tracer even if !thread_group_empty() but
	   do_wait(WEXITED) can't work until all threads exit

	- if the tracer is real_parent, it is not clear why can't
	  we use ->exit_signal event if !thread_group_empty()

-v2: do not try to fix the 2nd oddity to avoid the subtle behavior
     change mixed with reorganization, suggested by Tejun.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Tejun Heo <tj@kernel.org>

include/linux/tracehook.h

@@ -152,40 +152,6 @@ static inline void tracehook_signal_handler(int sig, siginfo_t *info,
 		ptrace_notify(SIGTRAP);
 }
 
-#define DEATH_REAP			-1
-#define DEATH_DELAYED_GROUP_LEADER	-2
-
-/**
- * tracehook_notify_death - task is dead, ready to notify parent
- * @task:		@current task now exiting
- * @death_cookie:	value to pass to tracehook_report_death()
- * @group_dead:		nonzero if this was the last thread in the group to die
- *
- * A return value >= 0 means call do_notify_parent() with that signal
- * number.  Negative return value can be %DEATH_REAP to self-reap right
- * now, or %DEATH_DELAYED_GROUP_LEADER to a zombie without notifying our
- * parent.  Note that a return value of 0 means a do_notify_parent() call
- * that sends no signal, but still wakes up a parent blocked in wait*().
- *
- * Called with write_lock_irq(&tasklist_lock) held.
- */
-static inline int tracehook_notify_death(struct task_struct *task,
-					 void **death_cookie, int group_dead)
-{
-	if (task_detached(task))
-		return task->ptrace ? SIGCHLD : DEATH_REAP;
-
-	/*
-	 * If something other than our normal parent is ptracing us, then
-	 * send it a SIGCHLD instead of honoring exit_signal.  exit_signal
-	 * only has special meaning to our real parent.
-	 */
-	if (thread_group_empty(task) && !ptrace_reparented(task))
-		return task->exit_signal;
-
-	return task->ptrace ? SIGCHLD : DEATH_DELAYED_GROUP_LEADER;
-}
-
 #ifdef TIF_NOTIFY_RESUME
 /**
  * set_notify_resume - cause tracehook_notify_resume() to be called

kernel/exit.c

@@ -819,9 +819,7 @@ static void forget_original_parent(struct task_struct *father)
  */
 static void exit_notify(struct task_struct *tsk, int group_dead)
 {
-	int signal;
 	bool autoreap;
-	void *cookie;
 
 	/*
 	 * This does two things:
@@ -852,16 +850,23 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 	 * we have changed execution domain as these two values started
 	 * the same after a fork.
 	 */
-	if (tsk->exit_signal != SIGCHLD && !task_detached(tsk) &&
+	if (thread_group_leader(tsk) && tsk->exit_signal != SIGCHLD &&
 	    (tsk->parent_exec_id != tsk->real_parent->self_exec_id ||
 	     tsk->self_exec_id != tsk->parent_exec_id))
 		tsk->exit_signal = SIGCHLD;
 
-	signal = tracehook_notify_death(tsk, &cookie, group_dead);
-	if (signal >= 0)
-		autoreap = do_notify_parent(tsk, signal);
-	else
-		autoreap = (signal == DEATH_REAP);
+	if (unlikely(tsk->ptrace)) {
+		int sig = thread_group_leader(tsk) &&
+				thread_group_empty(tsk) &&
+				!ptrace_reparented(tsk) ?
+			tsk->exit_signal : SIGCHLD;
+		autoreap = do_notify_parent(tsk, sig);
+	} else if (thread_group_leader(tsk)) {
+		autoreap = thread_group_empty(tsk) &&
+			do_notify_parent(tsk, tsk->exit_signal);
+	} else {
+		autoreap = true;
+	}
 
 	tsk->exit_state = autoreap ? EXIT_DEAD : EXIT_ZOMBIE;