linux-kernel - Re: [PATCH-RFC 2/2] eventfd: EFD

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Message-ID: <4A77FCB8.1000205@redhat.com>
Date:	Tue, 04 Aug 2009 12:17:44 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	davidel@...ilserver.org, gleb@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH-RFC 2/2] eventfd: EFD_STATE flag

On 08/04/2009 11:54 AM, Michael S. Tsirkin wrote:
> On Tue, Aug 04, 2009 at 11:53:03AM +0300, Avi Kivity wrote:
>    
>> On 08/03/2009 07:57 PM, Michael S. Tsirkin wrote:
>>      
>>>> Why not do it at the point of the write?
>>>>
>>>>       if (value != ctx->count) {
>>>>           ctx->count = value;
>>>>           wake_things_up();
>>>>       }
>>>>
>>>>          
>>> What if write comes before read?
>>>
>>>        
>> The read will get the new value.
>>      
>
> Yes :) But how does read know it should not block?
>    

If a different read comes after the write but after our read, it will 
have transferred the value, resulting in the same situation.

I think reads should never block with a state based mechanism.

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

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