On Wed, 2012-07-18 at 00:23 +0300, Michael S. Tsirkin wrote:
> On Tue, Jul 17, 2012 at 02:03:05PM -0600, Alex Williamson wrote:
> > On Tue, 2012-07-17 at 21:58 +0300, Michael S. Tsirkin wrote:
> > > On Tue, Jul 17, 2012 at 10:52:16AM -0600, Alex Williamson wrote:
> > > > On Tue, 2012-07-17 at 19:19 +0300, Michael S. Tsirkin wrote:
> > > > > On Tue, Jul 17, 2012 at 10:06:01AM -0600, Alex Williamson wrote:
> > > > > > On Tue, 2012-07-17 at 18:53 +0300, Michael S. Tsirkin wrote:
> > > > > > > On Tue, Jul 17, 2012 at 09:41:09AM -0600, Alex Williamson wrote:
> > > > > > > > On Tue, 2012-07-17 at 18:13 +0300, Michael S. Tsirkin wrote:
> > > > > > > > > On Tue, Jul 17, 2012 at 08:57:04AM -0600, Alex Williamson
> > > > > > > > > wrote:
> > > > > > > > > > On Tue, 2012-07-17 at 17:42 +0300, Michael S. Tsirkin wrote:
> > > > > > > > > > > On Tue, Jul 17, 2012 at 08:29:43AM -0600, Alex Williamson
> > > > > > > > > > > wrote:
> > > > > > > > > > > > On Tue, 2012-07-17 at 17:10 +0300, Michael S. Tsirkin
> > > > > > > > > > > > wrote:
> > > > > > > > > > > > > On Tue, Jul 17, 2012 at 07:59:16AM -0600, Alex
> > > > > > > > > > > > > Williamson wrote:
> > > > > > > > > > > > > > On Tue, 2012-07-17 at 13:21 +0300, Michael S.
> > > > > > > > > > > > > > Tsirkin wrote:
> > > > > > > > > > > > > > > On Mon, Jul 16, 2012 at 02:33:55PM -0600, Alex
> > > > > > > > > > > > > > > Williamson wrote:
> > > > > > > > > > > > > > > > + if (args->flags &
> > > > > > > > > > > > > > > > KVM_EOIFD_FLAG_LEVEL_IRQFD) {
> > > > > > > > > > > > > > > > + struct _irqfd *irqfd =
> > > > > > > > > > > > > > > > _irqfd_fdget_lock(kvm, args->irqfd);
> > > > > > > > > > > > > > > > + if (IS_ERR(irqfd)) {
> > > > > > > > > > > > > > > > + ret = PTR_ERR(irqfd);
> > > > > > > > > > > > > > > > + goto fail;
> > > > > > > > > > > > > > > > + }
> > > > > > > > > > > > > > > > +
> > > > > > > > > > > > > > > > + gsi = irqfd->gsi;
> > > > > > > > > > > > > > > > + level_irqfd =
> > > > > > > > > > > > > > > > eventfd_ctx_get(irqfd->eventfd);
> > > > > > > > > > > > > > > > + source =
> > > > > > > > > > > > > > > > _irq_source_get(irqfd->source);
> > > > > > > > > > > > > > > > + _irqfd_put_unlock(irqfd);
> > > > > > > > > > > > > > > > + if (!source) {
> > > > > > > > > > > > > > > > + ret = -EINVAL;
> > > > > > > > > > > > > > > > + goto fail;
> > > > > > > > > > > > > > > > + }
> > > > > > > > > > > > > > > > + } else {
> > > > > > > > > > > > > > > > + ret = -EINVAL;
> > > > > > > > > > > > > > > > + goto fail;
> > > > > > > > > > > > > > > > + }
> > > > > > > > > > > > > > > > +
> > > > > > > > > > > > > > > > + INIT_LIST_HEAD(&eoifd->list);
> > > > > > > > > > > > > > > > + eoifd->kvm = kvm;
> > > > > > > > > > > > > > > > + eoifd->eventfd = eventfd;
> > > > > > > > > > > > > > > > + eoifd->source = source;
> > > > > > > > > > > > > > > > + eoifd->level_irqfd = level_irqfd;
> > > > > > > > > > > > > > > > + eoifd->notifier.gsi = gsi;
> > > > > > > > > > > > > > > > + eoifd->notifier.irq_acked = eoifd_event;
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > OK so this means eoifd keeps a reference to the
> > > > > > > > > > > > > > > irqfd.
> > > > > > > > > > > > > > > And since this is the case, can't we drop the
> > > > > > > > > > > > > > > reference counting
> > > > > > > > > > > > > > > around source ids now? Everything is referenced
> > > > > > > > > > > > > > > through irqfd.
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Holding a reference and using it as a reference
> > > > > > > > > > > > > > count are not the same
> > > > > > > > > > > > > > thing. What if another module holds a reference to
> > > > > > > > > > > > > > this eventfd? How
> > > > > > > > > > > > > > do we do anything on release?
> > > > > > > > > > > > >
> > > > > > > > > > > > > We don't as there is no release, and using kref on
> > > > > > > > > > > > > source id does not
> > > > > > > > > > > > > help: it just never gets invoked.
> > > > > > > > > > > >
> > > > > > > > > > > > Please work out how you think it should work and let me
> > > > > > > > > > > > know, I don't
> > > > > > > > > > > > see it. We have an irq source id that needs to be
> > > > > > > > > > > > allocated by irqfd
> > > > > > > > > > > > and returned when it's unused. It becomes unused when
> > > > > > > > > > > > neither irqfd nor
> > > > > > > > > > > > eoifd are making use of it. irqfd and eoifd may be
> > > > > > > > > > > > closed in any order.
> > > > > > > > > > > > Use of the source id is what we're reference counting,
> > > > > > > > > > > > which is why it's
> > > > > > > > > > > > in struct _irq_source. How can I use an eventfd
> > > > > > > > > > > > reference for the same?
> > > > > > > > > > > > I don't know when it's unused. I don't know who else
> > > > > > > > > > > > holds a reference
> > > > > > > > > > > > to it... Doesn't make sense to me. Feels like
> > > > > > > > > > > > attempting to squat on
> > > > > > > > > > > > someone else's object.
> > > > > > > > > > > >
> > > > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > > > > eoifd should prevent irqfd from being released.
> > > > > > > > > >
> > > > > > > > > > Why? Note that this is actually quite difficult too. We
> > > > > > > > > > can't fail a
> > > > > > > > > > release, nobody checks close(3p) return. Blocking a
> > > > > > > > > > release is likely
> > > > > > > > > > to cause all sorts of problems, so what you mean is that
> > > > > > > > > > irqfd should
> > > > > > > > > > linger around until there are no references to it... but
> > > > > > > > > > that's exactly
> > > > > > > > > > what struct _irq_source is for, is to hold the bits that we
> > > > > > > > > > care about
> > > > > > > > > > references to and automatically release it when there are
> > > > > > > > > > none.
> > > > > > > > >
> > > > > > > > > No no. You *already* prevent it. You take a reference to the
> > > > > > > > > eventfd
> > > > > > > > > context.
> > > > > > > >
> > > > > > > > Right, which keeps the fd from going away, not the struct
> > > > > > > > _irqfd.
> > > > > > >
> > > > > > > _irqfd too.
> > > > > >
> > > > > >
> > > > > > How so?
> > > > >
> > > > > Normally irqfd_wakeup is called with POLLHUP and calls
> > > > > irqfd_deactivate.
> > > > > If you get a ctx reference this does not happen.
> > > >
> > > > I think you're mistaken. wake_up_poll(,POLLHUP) is called from
> > > > eventfd_release (file_operations.release), not from ctx reference
> > > > release.
> > >
> > > True. I was wrong. so close has the same bug as deassign. To fix,
> > > how about eoifd will hold a reference to the irqfd instead of the
> > > eventfd context?
> >
> > What does it mean to hold a reference to the irqfd?
>
> I meant file *reference: eventfd_fget. But there are other options see
> below.
That's no better than the eventfd context we already hold.
> > What state of functionality is an irqfd that has been
> > closed/de-assigned but is still attached to an eoifd? It can't
> > continue to fire interrupts into the guest.
> >
> > I don't think close or de-assign have a bug, assign has a bug that it
> > can allow re-assignment using an in-use eventfd. I think I'd rather
> > fix that.
>
> Let me show you that the bug is in deassign:
> assign irqfd for fd=1
> assign for eoifd fd=2, irqfd=1
> deassign irqfd 1
>
> At this point eoifd has no meaning and there is also no way to deassign
> it,
Yes, there is. This is exactly why I hold a reference to the eventfd
ctx. It can still be deassigned by passing irqfd=1, we'll do an
eventfd_ctx_get and match it to that stored.
> so the bug already triggered.
>
> I can see two ways out:
> 1. easy way - fail deassign
Then close() and deassign are not the same.
> 2. elegant way - shut down eoifd on irqfd deassign too
Sorry, I've always been told it's a bad idea to have one interface kill
another from inside the kernel.
Given that your assertion above is incorrect, I still stand by fixing
assign.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
