move exit_task_work() past exit_files() et.al.

... and get rid of PF_EXITING check in task_work_add().
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/exit.c

@@ -953,14 +953,11 @@ void do_exit(long code)
 	exit_signals(tsk);  /* sets PF_EXITING */
 	/*
 	 * tsk->flags are checked in the futex code to protect against
-	 * an exiting task cleaning up the robust pi futexes, and in
-	 * task_work_add() to avoid the race with exit_task_work().
+	 * an exiting task cleaning up the robust pi futexes.
 	 */
 	smp_mb();
 	raw_spin_unlock_wait(&tsk->pi_lock);
 
-	exit_task_work(tsk);
-
 	if (unlikely(in_atomic()))
 		printk(KERN_INFO "note: %s[%d] exited with preempt_count %d\n",
 				current->comm, task_pid_nr(current),
@@ -995,6 +992,7 @@ void do_exit(long code)
 	exit_shm(tsk);
 	exit_files(tsk);
 	exit_fs(tsk);
+	exit_task_work(tsk);
 	check_stack_usage();
 	exit_thread();
 

kernel/task_work.c

@@ -5,34 +5,26 @@
 int
 task_work_add(struct task_struct *task, struct callback_head *twork, bool notify)
 {
+	struct callback_head *last, *first;
 	unsigned long flags;
-	int err = -ESRCH;
 
-#ifndef TIF_NOTIFY_RESUME
-	if (notify)
-		return -ENOTSUPP;
-#endif
 	/*
-	 * We must not insert the new work if the task has already passed
-	 * exit_task_work(). We rely on do_exit()->raw_spin_unlock_wait()
-	 * and check PF_EXITING under pi_lock.
+	 * Not inserting the new work if the task has already passed
+	 * exit_task_work() is the responisbility of callers.
 	 */
 	raw_spin_lock_irqsave(&task->pi_lock, flags);
-	if (likely(!(task->flags & PF_EXITING))) {
-		struct callback_head *last = task->task_works;
-		struct callback_head *first = last ? last->next : twork;
-		twork->next = first;
-		if (last)
-			last->next = twork;
-		task->task_works = twork;
-		err = 0;
-	}
+	last = task->task_works;
+	first = last ? last->next : twork;
+	twork->next = first;
+	if (last)
+		last->next = twork;
+	task->task_works = twork;
 	raw_spin_unlock_irqrestore(&task->pi_lock, flags);
 
 	/* test_and_set_bit() implies mb(), see tracehook_notify_resume(). */
-	if (likely(!err) && notify)
+	if (notify)
 		set_notify_resume(task);
-	return err;
+	return 0;
 }
 
 struct callback_head *