linux-kernel - Re: [PATCH v5 2/4] kvm: KVM

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Date:	Tue, 17 Jul 2012 07:59:16 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	avi@...hat.com, gleb@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, jan.kiszka@...mens.com
Subject: Re: [PATCH v5 2/4] kvm: KVM_EOIFD, an eventfd for EOIs

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?

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