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

From: Davide Libenzi <davidel@xmailserver.org>
To: Gregory Haskins <ghaskins@novell.com>
Cc: 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>,
	Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [PATCH 3/3] eventfd: add internal reference counting to fix notifier race conditions
Date: Mon, 22 Jun 2009 11:40:52 -0700 (PDT)	[thread overview]
Message-ID: <alpine.DEB.1.10.0906221132210.10952@makko.or.mcafeemobile.com> (raw)
In-Reply-To: <4A3FCDF2.3010903@novell.com>

On Mon, 22 Jun 2009, Gregory Haskins wrote:

> The general thesis is for decoupling of the two subsystems.  In order to
> do this, you need some form of polymorphism and an intermediate "handle"
> mechanism which is userspace friendly.  File-descriptors already fit
> this role neatly, with the "int fd" being the handle, and the f_ops
> being the polymorphic interface.  Eventfd is of course, a subclass of
> this concept in that it has these same general properties but with
> signaling semantics (non-blocking collapsible events, etc).
> 
> Say, for example, you wanted disk IO completion events to generate an
> interrupt into a guest.  One way to do this would, of course, modify all
> the disk-io code so it knows how to directly inject a KVM guest
> interrupt.   While this would work, someone would undoubtedly get flamed
> for such a suggestion ;)
> 
> Another way to do it is to treat the AIO eventfd as the hook point. 
> IIUC AIO already knows how to be an eventfd producer.  KVM, by virtue of
> irqfd, already knows how to be an eventfd consumer.  So now kvm can
> consume AIO, or it can consume userspace events equally well, and
> without modification.  Neither side needs to know about the other per
> se, other than the details on how to use the eventfd interface.
> 
> Don't get me wrong:  We expect userspace to use all this stuff too.  I
> just expect that we will see all permutations of producer/consumer +
> userspace/kernel combinations, so I want to retain that "all producers
> have left" notification feature set.  Today eventfd supports producers
> or consumers in userspace, and producers in the kernel.  This new work
> we are doing adds consumer support in the kernel.  Kernel to kernel is
> just a natural extension of that.

A file* is the VFS link between userspace and the kernel. Is not a magical 
polymorphic interface to be used for whatever kernel side reasons.
Basing a kernel internal API over it is flawed.
On top of that, a single reference count does not put you on cover about 
the possible combinations of producers and consumers. For that, you'd need 
a pipe-like reference handling logic, that is way far from the eventfd scope.
So please stop making hypothetical cases about interface usages, and 
*when* we will have a real case, we'll see what the better handling for it 
will be.

- Davide