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

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Date:	Wed, 15 Aug 2007 00:01:54 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	paulmck@...ux.vnet.ibm.com
Cc:	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 Tuesday 14 August 2007, Paul E. McKenney wrote:
> > #define order(x) asm volatile("" : "+m" (x))
> 
> There was something very similar discussed earlier in this thread,
> with quite a bit of debate as to exactly what the "m" flag should
> look like.  I suggested something similar named ACCESS_ONCE in the
> context of RCU (http://lkml.org/lkml/2007/7/11/664):
> 
>         #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
> 
> The nice thing about this is that it works for both loads and stores.
> Not clear that order() above does this -- I get compiler errors when
> I try something like "b = order(a)" or "order(a) = 1" using gcc 4.1.2.

Well, it serves a different purpose: While your ACCESS_ONCE() macro is
an lvalue, the order() macro is a statement that can be used in place
of the barrier() macro. order() is the most lightweight barrier as it
only enforces ordering on a single variable in the compiler, but does
not have any side-effects visible to other threads, like the cache
line access in ACCESS_ONCE has.

	Arnd <><
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