Re: [PATCH-RFC 2/2] eventfd: EFD_STATE flag

Gleb Natapov
2009-08-04T09:17:58Z

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.