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

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Message-ID: <20090804091755.GD4764@redhat.com>
Date:	Tue, 4 Aug 2009 12:17:55 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>, davidel@...ilserver.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH-RFC 2/2] eventfd: EFD_STATE flag

On Tue, Aug 04, 2009 at 12:17:44PM +0300, Avi Kivity wrote:
> 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.
>
Reader may want to poll for the status change.

--
			Gleb.
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