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

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Message-ID: <4B5C29BC.4050109@redhat.com>
Date:	Sun, 24 Jan 2010 13:06:36 +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 08:02 PM, Michael S. Tsirkin wrote:
> On Thu, Jan 21, 2010 at 07:57:22PM +0200, Avi Kivity wrote:
>    
>> 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.
>>      
> I would rather not require that userspace never reads this fd.
> You are right that it does not now, but adding this as requirement
> looks like exporting an implementation bug to userspace.
>    

Well, I don't like risking 2.6.32 non-kvm users for a bug that doesn't 
happen in practice now.

After it gets some time in 2.6.33, we can backport it to 2.6.32 (since 
that will be maintained long term).

-- 
error compiling committee.c: too many arguments to function

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