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

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Message-ID: <alpine.DEB.2.00.0908241508440.7951@makko.or.mcafeemobile.com>
Date:	Mon, 24 Aug 2009 15:15:05 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
cc:	Avi Kivity <avi@...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 Tue, 25 Aug 2009, Michael S. Tsirkin wrote:

> On Mon, Aug 24, 2009 at 11:25:01AM -0700, Davide Libenzi wrote:
> > On Sun, 23 Aug 2009, Michael S. Tsirkin wrote:
> > 
> > > On Sun, Aug 23, 2009 at 04:40:51PM +0300, Avi Kivity wrote:
> > > > On 08/23/2009 04:36 PM, Michael S. Tsirkin wrote:
> > > >> More important here is realization that eventfd is a mutex/semaphore
> > > >> implementation, not a generic event reporting interface as we are trying
> > > >> to use it.
> > > >>    
> > > >
> > > > Well it is a generic event reporting interface (for example, aio uses it).
> > > 
> > > Davide, I think it's a valid point.  For example, what read on eventfd
> > > does (zero a counter and return) is not like any semaphore I saw.
> > 
> > 
> > Indeed, the default eventfd behaviour is like, well, an event. Signaling 
> > (kernel side) or writing (userspace side), signals the event.
> > Waiting (reading) it, will reset the event.
> > If you use EFD_SEMAPHORE, you get a semaphore-like behavior.
> > Events and sempahores are two widely known and used abstractions.
> > The EFD_STATE proposed one, well, no. Not at all.
> 
> Hmm. All we try to do is, associate a small key with the event
> that we signal. Is it really that uncommon/KVM specific?

All I'm trying to do, is to avoid that eventfd will become an horrible 
multiplexor for every freaky one-time-use behaviors arising inside kernel 
modules.



- Davide


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