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

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Message-ID: <4A27B37F.1060307@gmail.com>
Date:	Thu, 04 Jun 2009 07:43:59 -0400
From:	Gregory Haskins <gregory.haskins@...il.com>
To:	Avi Kivity <avi@...hat.com>
CC:	"Michael S. Tsirkin" <mst@...hat.com>,
	Gregory Haskins <ghaskins@...ell.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, davidel@...ilserver.org,
	paulmck@...ux.vnet.ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [KVM-RFC PATCH 0/2] irqfd: use POLLHUP notification for close()

Avi Kivity wrote:
> Michael S. Tsirkin wrote:
>> 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.
>>   
>
> I agree that deassign is needed for reasons of symmetry, and that it
> can be added later.
>
Cool.

FYI: Davide's patch has been accepted into -mm (Andrew CC'd).  I am not
sure of the protocol here, but I assume this means you can now safely
pull it from -mm into kvm.git so the prerequisite for 2/2 is properly met.

-Greg


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