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

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Message-ID: <alpine.DEB.1.10.0906191525470.14884@makko.or.mcafeemobile.com>
Date:	Fri, 19 Jun 2009 15:47:11 -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, Davide Libenzi wrote:

> 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?

AFAICS, the IRQfd code simply registers the callback to ->poll() and waits 
for two events.
In the POLLIN event, you schedule_work(&irqfd->inject) and there are no 
races there AFAICS (you basically do not care of anything eventfd memory 
related at all).
For POLLHUP, you do:

	spin_lock(irqfd->slock);
	if (irqfd->wqh)
		schedule_work(&irqfd->inject);
	irqfd->wqh = NULL;
	spin_unlock(irqfd->slock);

In your work function you notice the POLLHUP condition and take proper 
action (dunno what it is in your case).
In your kvm_irqfd_release() function:

	spin_lock(irqfd->slock);
	if (irqfd->wqh)
		remove_wait_queue(irqfd->wqh, &irqfd->wait);
	irqfd->wqh = NULL;
	spin_unlock(irqfd->slock);

Any races in there?



- Davide


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