linux-kernel - Re: [patch] eventfd - revised interface and cleanups (2nd rev)

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Date:	Tue, 23 Jun 2009 12:49:02 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	avi@...hat.com, kvm@...r.kernel.org, ghaskins@...ell.com,
	Rusty Russell <rusty@...tcorp.com.au>,
	Benjamin LaHaise <bcrl@...ck.org>
Subject: Re: [patch] eventfd - revised interface and cleanups (2nd rev)

On Tue, 23 Jun 2009, Andrew Morton wrote:

> On Tue, 23 Jun 2009 12:25:36 -0700 (PDT)
> Davide Libenzi <davidel@...ilserver.org> wrote:
> 
> > The following patch changes the eventfd interface to de-couple the eventfd 
> > memory context, from the file pointer instance.
> > Without such change, there is no clean way to racely free handle the 
> > POLLHUP event sent when the last instance of the file* goes away.
> > Also, now the internal eventfd APIs are using the eventfd context instead 
> > of the file*.
> > Another cleanup this patch does, is making AIO select EVENTFD, instead of 
> > adding a bunch of empty function stubs inside eventfd.h in order to 
> > handle the (AIO && !EVENTFD) case.
> > 
> 
> If we really want to squeeze this into 2.6.31 then it would be helpful
> to have justification in the changelog, please.  I see that some KVM
> feature needs it, but what and why and why now, etc?

The patch in -next added the ability to have waiters to be notified when 
the last instance of the file* is dropped (that is, on ->release).
But it is not possible for waiters to handle the POLLHUP event in a 
racy-free way, unless the eventfd memory context is de-coupled from the 
file*.
The next patches from gregory will use eventfd (and the POLLHUP handling) 
to implement the IRQfd code inside KVM.


> What's going on here?

The POLLHUP patch in -next is not sufficent. I based the patch over 
mainline, but if you guys want I can rebase it over -next.



- Davide


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