Subject: Re: Getting the new RxRPC patches upstream

From: David Howells <[email-masked]>
Date: Tue, 24 Apr 2007 19:22:50 +0100

Oleg Nesterov <[email-masked]> wrote:

> Sure, I'll grep for cancel_delayed_work(). But unless I missed something,
> this change should be completely transparent for all users. Otherwise, it
> is buggy.

I guess you will have to make sure that cancel_delayed_work() is always
followed by a flush of the workqueue, otherwise you might get this situation:

	CPU 0				CPU 1
	===============================	=======================
					<timer expires>
	cancel_delayed_work(x) == 0	-->delayed_work_timer_fn(x)
	kfree(x);			-->do_IRQ()
	y = kmalloc(); // reuses x
					<--do_IRQ()
					__queue_work(x)
	--- OOPS ---

That's my main concern.  If you are certain that can't happen, then fair
enough.

Note that although you can call cancel_delayed_work() from within a work item
handler, you can't then follow it up with a flush as it's very likely to
deadlock.

> > Because calling schedule_delayed_work() is a waste of CPU if the timer
> > expiry handler is currently running at this time as *that* is going to
> > also schedule the delayed work item.
> 
> Yes. But otoh, try_to_del_timer_sync() is a waste of CPU compared to
> del_timer(), when the timer is not pending.

I suppose that's true.  As previously stated, my main objection to del_timer()
is the fact that it doesn't tell you if the timer expiry function is still
running.

Can you show me a patch illustrating exactly how you want to change
cancel_delayed_work()?  I can't remember whether you've done so already, but
if you have, I can't find it.  Is it basically this?:

 static inline int cancel_delayed_work(struct delayed_work *work)
 {
 	int ret;

-	ret = del_timer_sync(&work->timer);
+	ret = del_timer(&work->timer);
 	if (ret)
 		work_release(&work->work);
 	return ret;
 }

I was thinking this situation might be a problem:

	CPU 0				CPU 1
	===============================	=======================
					<timer expires>
	cancel_delayed_work(x) == 0	-->delayed_work_timer_fn(x)
	schedule_delayed_work(x,0)	-->do_IRQ()
	<keventd scheduled>
	x->work()
					<--do_IRQ()
					__queue_work(x)

But it won't, will it?

> > Ah, but the timer routine may try to set the work item pending flag
> > *after* the work_pending() check you have here.
> 
> No, delayed_work_timer_fn() doesn't set the _PENDING flag.

Good point.  I don't think that's a problem because cancel_delayed_work()
won't clear the pending flag if it didn't remove a timer.

> First, this is very unlikely event, delayed_work_timer_fn() is very fast
> unless interrupted.

Yeah, I guess so.

Okay, you've convinced me, I think - provided you consider the case I
outlinded at the top of this email.

If you give me a patch to alter cancel_delayed_work(), I'll substitute it for
mine and use that that instead.  Dave Miller will just have to live with that
patch being there:-)

David