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

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Date:	Wed, 24 Jun 2009 12:55:54 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Gregory Haskins <ghaskins@...ell.com>, 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 Tue, 23 Jun 2009 03:33:22 am Davide Libenzi wrote:
> What you're doing there, is setting up a kernel-to-kernel (since
> userspace only role is to create the eventfd) communication, using a file*
> as accessory. That IMO is plain wrong.

The most sensible is that userspace can use these fds; an in-kernel variant is 
possible too, but not primary IMHO.

It's nice that userspace create the fds; it can then use the same fd for 
multiple event sources.

But I didn't see anything wrong with the way eventfd used to work: you have a 
kvm ioctl to say "attach this eventfd to this guest notification" and that does 
the eventfd_fget.  A detach ioctl does the fput (as does release of the kvm 
fd).

If they close the eventfd and don't do the detach ioctl, it's their problem.

Rusty.
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