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

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Message-ID: <20090804092342.GE4764@redhat.com>
Date:	Tue, 4 Aug 2009 12:23:42 +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 12:25:49PM +0300, Avi Kivity wrote:
> On 08/04/2009 12:17 PM, Gleb Natapov wrote:
>>> 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.
>>    
>
> Without epoll(), it's inherently racy since reads from other processes  
> can clear the status.
>
This is correct for any file descriptor. Multiple readers shouldn't
simultaneously read from the same files descriptor if they expect to
make any sense from a result.

> The "last read value" needs to be maintained for each reader, which is  
> not possible with read().
>
Only one reader scenario is interesting. This is not some multiplexing
device.

--
			Gleb.
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