Re: [PATCH 0/2] eventfd: new EFD_STATE flag

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Jan 10, 2010 at 09:27:34AM -0800, Davide Libenzi wrote:
> On Sun, 10 Jan 2010, Michael S. Tsirkin wrote:
> 
> > Not sure what you mean by irqfd locking.  IMO we must take waitqueue
> > lock, otherwise we can not prevent wait queue callback from being
> > invoked, right? Note that if we take our own lock under wait queue
> > callback, we won't be able to use this lock around remove_wait_queue.
> > Also note how fs/evetfd.c uses __remove_wait_queue after taking wait
> > queue lock - we'd like to do the same with our wait queue.
> > 
> > Could you clarify pls?
> 
> Wait a second. Looking at the current code in Linus tree, when you 
> deassign an irqfd, you are actually destroying it, so the idea of saving 
> the counter on deassign is not going to work.
> 
> 
> 
> - Davide

Yes, the only context passed between deassign and assign is the eventfd
itself. So I think the following code for deassign would work provided
eventfd_ctx_read_locked works with wait queue lock taken:

        spin_lock_irqsave(&irqfd->wqh.lock, flags);
        eventfd_ctx_read_locked(ctx->eventfd, 1, &ucnt);
        __remove_wait_queue(irqfd->wqh, &irqfd->wait);
        spin_lock_irqrestore(&irqfd->wqh.lock, flags);

And we discard ucnt here. This works because on on assign we do:

	events = file->f_op->poll(file, &irqfd->pt);
	if (events & POLLIN)
		schedule_work(&irqfd->inject);

-- 
MST