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

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Message-ID: <alpine.DEB.2.00.0908270732470.3809@makko.or.mcafeemobile.com>
Date:	Thu, 27 Aug 2009 07:38:46 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
cc:	Paolo Bonzini <pbonzini@...hat.com>, 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 Thu, 27 Aug 2009, Michael S. Tsirkin wrote:

> On Thu, Aug 27, 2009 at 07:21:49AM -0700, Davide Libenzi wrote:
> > On Thu, 27 Aug 2009, Paolo Bonzini wrote:
> > 
> > > > Ok, so why not using the eventfd counter as state?
> > > > On the device side:
> > > > 
> > > > void write_state(int sfd, int state) {
> > > > 	u64 cnt;
> > > > 
> > > > 	/* Clear the current state, sfd is in non-blocking mode */
> > > > 	read(sfd,&cnt, sizeof(cnt));
> > > > 	/* Writes new state */
> > > > 	cnt = 1 + !!state;
> > > > 	write(sfd,&cnt, sizeof(cnt));
> > > > }
> > > 
> > > It's interesting [no sarcasm intended, mind] that EFD_SEMAPHORE was
> > > added exactly to avoid a read+write combination for the case of
> > > decrementing a value.
> > 
> > Like I repeated 25 times already, EFD_SEMAPHORE was added, because a 
> > *semaphore* is a pretty widely known and used abstraction.
> 
> what about an atomic variable, btw?  does it make sense to implement
> write that does compare and exchange?

It is surprising to me, that is front of a workable solution w/out any 
use-once additions, yet you want to try to add optimizations and new 
ad-hoc abstractions to user visible interfaces.
Now, you tell me what an atomic variable has to do with an eventfd.


- Davide


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