linux-kernel - Re: [PATCH 3/3] eventfd: add internal reference counting to fix notifier race conditions

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Message-ID: <4A3FD89A.8080809@gmail.com>
Date:	Mon, 22 Jun 2009 15:16:42 -0400
From:	Gregory Haskins <gregory.haskins@...il.com>
To:	Davide Libenzi <davidel@...ilserver.org>
CC:	"Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	avi@...hat.com, paulmck@...ux.vnet.ibm.com,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 3/3] eventfd: add internal reference counting to fix notifier
 race conditions

Davide Libenzi wrote:
> On Mon, 22 Jun 2009, Michael S. Tsirkin wrote:
>
>   
>> On Mon, Jun 22, 2009 at 11:03:22AM -0700, Davide Libenzi wrote:
>>     
>>> In your case of kernel-to-kernel scenario, why would you need eventfd at 
>>> all, if userspace role in that model is simply to create it?
>>>       
>> That's not 100% true. We have a mode where userspace is the producer
>> and/or consumer (migration mode) and we switch between that and
>> direct kernel-to-kernel communication.
>>     
>
> Then you'd need to ask yourself how to handle your complex case inside the 
> KVM code, so that other eventfd users are not affected by the extra fat 
> needed to handle your scenarios. Thing that seem to be continuosly tried.
> A file* based kernel-to-kernel interface is rather wrong IMO.
>   

Well, I will point out that the interface in question is
eventfd_signal(struct file *), and you were the one that invented it
afaict.  Can't help it if we like it :)

BTW: The termination point of that call is incidental.  Afterall, it
works the same for kernel-kernel or kernel-userspace.  The only relevant
discussion point here is that your proposal breaks POLLHUP for
eventfd_signal() users, and we have a real use case that cares.

Let me ask the question a different way: Lets say we all agree that
eventfd_signal() cannot be file* based to be correct.  Is there a way
you can think of that satisfies both the need to get rid of file* AND
still deliver producer-release notification to consumers.  Ultimately
that is all I care about.

-Greg

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