linux-kernel - Re: [PATCH 0/2] eventfd: new EFD

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Date:	Thu, 20 Aug 2009 19:56:05 +0200
From:	Paolo Bonzini <bonzini@....org>
To:	Davide Libenzi <davidel@...ilserver.org>
CC:	Avi Kivity <avi@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>,
	gleb@...hat.com, kvm@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] eventfd: new EFD_STATE flag

On 08/20/2009 07:44 PM, Davide Libenzi wrote:
> On Thu, 20 Aug 2009, Avi Kivity wrote:
>
>> On 08/20/2009 07:20 PM, Davide Libenzi wrote:
>>>
>>> I briefly looked at this while in vacation, although I did not reply
>>> hoping the horrible feeling about this code would go away.
>>> It didn't.
>>> I find this to be an ugly and ad-hoc multiplexing of eventfd with added
>>> functionalities of questionable general use.
>>> I'm pretty sure you can do better on KVM side, to solve the problem w/out
>>> littering eventfd.
>>
>> While we could argue about this my feeling is that we should drop this, at
>> least until we can quantify what benefit it has and whether there are any
>> Davide-acceptable alternatives.
>
> I really didn't mean to be harsh, but seriously, we cannot just have a
> Multiplexing Feast over eventfd, with one-time users.

EFD_STATE actually does two changes: a) makes read block until the value 
changes; b) makes each write override the previous one.  How would you 
feel if the two changes were separated?  I can see each of them has use 
cases

For example, (a) could be implemented by using select's xfds (POLLPRI) 
to poll for value changes (rfds would still poll for non-zeroness). 
Then Michael does not need even to read the eventfd; instead he'd check 
POLLIN with a zero timeout.

(b) could be implemented with a flag like Michael did.

Paolo
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