linux-kernel - Re: [KVM-RFC PATCH 0/2] irqfd: use POLLHUP notification for close()

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Date:	Wed, 3 Jun 2009 09:39:56 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com,
	davidel@...ilserver.org, paulmck@...ux.vnet.ibm.com
Subject: Re: [KVM-RFC PATCH 0/2] irqfd: use POLLHUP notification for close()

On Tue, Jun 02, 2009 at 01:41:05PM -0400, Gregory Haskins wrote:
> > And having close not clean up the state unless you do an ioctl first is
> > very messy IMO - I don't think you'll find any such examples in kernel.
> >
> >   
> 
> I agree, and that is why I am advocating this POLLHUP solution.  It was
> only this other way to begin with because the technology didn't exist
> until Davide showed me the light.
> 
> Problem with your request is that I already looked into what is
> essentially a bi-directional reference problem (for a different reason)
> when I started the POLLHUP series.  Its messy to do this in a way that
> doesn't negatively impact the fast path (introducing locking, etc) or
> make my head explode making sure it doesn't race.  Afaict, we would need
> to solve this problem to do what you are proposing (patches welcome).
> 
> If this hybrid decoupled-deassign + unified-close is indeed an important
> feature set, I suggest that we still consider this POLLHUP series for
> inclusion, and then someone can re-introduce DEASSIGN support in the
> future as a CAP bit extension.  That way we at least get the desirable
> close() properties that we both seem in favor of, and get this advanced
> use case when we need it (and can figure out the locking design).
> 

FWIW, I took a look and yes, it is non-trivial.
I concur, we can always add the deassign ioctl later.



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MST
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