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

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Message-ID: <alpine.DEB.1.10.0906191447330.14884@makko.or.mcafeemobile.com>
Date:	Fri, 19 Jun 2009 14:54:20 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Gregory Haskins <ghaskins@...ell.com>
cc:	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: [PATCH 3/3] eventfd: add internal reference counting to fix
 notifier race conditions

On Fri, 19 Jun 2009, Gregory Haskins wrote:

> I am fairly confident it is not that simple after having thought about
> this issue over the last few days.  But I've been wrong in the past. 
> Propose a patch and I will review it for races/correctness, if you
> like.  Perhaps a combination of that plus your asymmetrical locking
> scheme would work.  One of the challenges you will hit is avoiding ABBA
> between your "get" lock and the wqh, but good luck!

A patch for what? The eventfd patch is a one-liner.
It seems hard to believe that the thing cannot be handled on your side. 
Once the wake_up_locked() is turned into a wake_up(), what other races are 
there?
Let's not try to find the problem that fits and justify the "cool" 
solution, but let's see if a problem is there at all.


- Davide


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