netdev - Re: [PATCH 6/24] make atomic_read() behave consistently on frv

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Message-Id: <200708151706.48974.mb@bu3sch.de>
Date:	Wed, 15 Aug 2007 17:06:47 +0200
From:	Michael Buesch <mb@...sch.de>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	paulmck@...ux.vnet.ibm.com, Nick Piggin <nickpiggin@...oo.com.au>,
	Herbert Xu <herbert@...dor.apana.org.au>, csnook@...hat.com,
	dhowells@...hat.com, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, torvalds@...ux-foundation.org,
	netdev@...r.kernel.org, akpm@...ux-foundation.org, ak@...e.de,
	heiko.carstens@...ibm.com, davem@...emloft.net,
	schwidefsky@...ibm.com, wensong@...ux-vs.org, horms@...ge.net.au,
	wjiang@...ilience.com, cfriesen@...tel.com, zlynx@....org,
	rpjday@...dspring.com, jesper.juhl@...il.com
Subject: Re: [PATCH 6/24] make atomic_read() behave consistently on frv

On Wednesday 15 August 2007 15:29:43 Arnd Bergmann wrote:
> On Wednesday 15 August 2007, Paul E. McKenney wrote:
> 
> > ACCESS_ONCE() is indeed intended to be used when actually loading or
> > storing the variable.  That said, I must admit that it is not clear to me
> > why you would want to add an extra order() rather than ACCESS_ONCE()ing
> > one or both of the adjacent accesses to that same variable.
> > 
> > So, what am I missing?
> 
> You're probably right, the only case I can construct is something like
> 
> 	if (ACCESS_ONCE(x)) {
> 		...
> 		ACCESS_ONCE(x)++;
> 	}
> 
> which would be slightly less efficient than
> 
> 	if (x)
> 		x++;
> 	order(x);
> 
> because in the first case, you need to do two ordered accesses
> but only one in the second case. However, I can't think of a case
> where this actually makes a noticable difference in real life.

How can this example actually get used in a sane and race-free
way? This would need locking around the whole if
statement. But locking is a barrier.

-- 
Greetings Michael.
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