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

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Message-ID: <4A265FBD.3080108@novell.com>
Date:	Wed, 03 Jun 2009 07:34:21 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	"Michael S. Tsirkin" <mst@...hat.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()

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.
>
>
>
>   

Sounds good, Michael.  Thanks!

-Greg


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