Message-ID: <4A37CFDA.4000602@novell.com>
Date: Tue, 16 Jun 2009 13:01:14 -0400
From: Gregory Haskins <ghaskins@novell.com>
User-Agent: Thunderbird 2.0.0.21 (Macintosh/20090302)
MIME-Version: 1.0
To: Davide Libenzi <davidel@xmailserver.org>
CC: "Michael S. Tsirkin" <mst@redhat.com>, kvm@vger.kernel.org,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       avi@redhat.com, paulmck@linux.vnet.ibm.com, Ingo Molnar <mingo@elte.hu>
Subject: Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier
 interface
References: <20090616022041.23890.90120.stgit@dev.haskins.net> <20090616022956.23890.63776.stgit@dev.haskins.net> <20090616140240.GA9401@redhat.com> <4A37A7FC.4090403@novell.com> <20090616143816.GA18196@redhat.com> <4A37B0BB.3020005@novell.com> <20090616145502.GA1102@redhat.com> <4A37B832.6040206@novell.com> <20090616154150.GA17494@redhat.com> <4A37C592.2030407@novell.com> <alpine.DEB.1.10.0906160915340.25664@makko.or.mcafeemobile.com>
In-Reply-To: <alpine.DEB.1.10.0906160915340.25664@makko.or.mcafeemobile.com>
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Davide Libenzi wrote:
> On Tue, 16 Jun 2009, Gregory Haskins wrote:
>
>  =20
>> Does this all make sense?
>>    =20
>
> This conversation has been *really* long, and I haven't had time to loo=
k=20
> at the patch yet. But looking at the amount of changes, and the amount =
of=20
> even more changes talked in this thread, there's a very slim chance tha=
t=20
> I'll ACK the eventfd code.
> You may want to consider a solution that does not litter eventfd code t=
hat=20
> much.
>
>
> - Davide
>
>
>  =20
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).=20
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.=20
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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