linux-kernel - Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier interface

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Message-ID: <4A37CFDA.4000602@novell.com>
Date:	Tue, 16 Jun 2009 13:01:14 -0400
From:	Gregory Haskins <ghaskins@...ell.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>
Subject: Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier
 interface

Davide Libenzi wrote:
> On Tue, 16 Jun 2009, Gregory Haskins wrote:
>
>   
>> Does this all make sense?
>>     
>
> This conversation has been *really* long, and I haven't had time to look 
> at the patch yet. But looking at the amount of changes, and the amount of 
> even more changes talked in this thread, there's a very slim chance that 
> I'll ACK the eventfd code.
> You may want to consider a solution that does not litter eventfd code that 
> much.
>
>
> - Davide
>
>
>   
Hi Davide,

I understand your position and value your time/insight into looking at
this things.

Despite the current ongoing discussion, I still stand that the current
patch is my proposed solution (though I have yet to convince Michael). 
But in any case,  if you have the time, please look it over because I
still think its the right direction to head in.

The general solution is that we use an srcu list instead of the
wait-queue, and thats really it.  If we can't eliminate that spinlock
held over the notification, it has usability implications at least for
irqfd/iosignalfd.  The only way I can think of to solve the problem
without modifying eventfd is to not use eventfd at all.   :(

Since using eventfd really captures the concept we are going for here
really well, reusing it has a ton of advantages including interface
compatibility and, of course, code-reuse of a tested/debugged code
base.  Heck, we may hopefully even improve eventfd for other users in
doing this work.  It would therefore be a shame to walk away from it if
it can be avoided.

So if what I proposed is not acceptable but you are willing to work with
me to find a solution that is, that would be ideal from my perspective. 
Otherwise, I apologize for all the noise.  You have been quite the
patient and helpful gentleman with me to date and I appreciate that.

Kind Regards,
-Greg

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