linux-kernel - Re: [PATCHv2 1/3] eventfd: allow atomic read and waitqueue remove

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Message-ID: <4B589582.9030300@redhat.com>
Date:	Thu, 21 Jan 2010 19:57:22 +0200
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Davide Libenzi <davidel@...ilserver.org>, mtosatti@...hat.com,
	kvm@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv2 1/3] eventfd: allow atomic read and waitqueue remove

On 01/21/2010 07:56 PM, Avi Kivity wrote:
> On 01/21/2010 07:45 PM, Michael S. Tsirkin wrote:
>>
>>> But you're in process context.  An eventfd never blocks.
>> Yes it blocks if counter is 0. And we don't know
>> it's not 0 unless we read :) catch-22.
>
> Ah yes, I forgot.
>

Well, you can poll it and then read it... this introduces a new race (if 
userspace does a read in parallel) but it's limited to kvm and buggy 
userspace.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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