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

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Message-ID: <alpine.DEB.2.00.0908270720250.3809@makko.or.mcafeemobile.com>
Date:	Thu, 27 Aug 2009 07:21:49 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Paolo Bonzini <pbonzini@...hat.com>
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 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.


- Davide


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