linux-kernel - Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

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Message-Id: <20070815233721.91032366.billfink@mindspring.com>
Date:	Wed, 15 Aug 2007 23:37:21 -0400
From:	Bill Fink <billfink@...dspring.com>
To:	Satyam Sharma <satyam@...radead.org>
Cc:	Stefan Richter <stefanr@...6.in-berlin.de>,
	Christoph Lameter <clameter@....com>,
	Chris Snook <csnook@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	netdev@...r.kernel.org, Andrew Morton <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,
	segher@...nel.crashing.org,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
 architectures

On Wed, 15 Aug 2007, Satyam Sharma wrote:

> (C)
> $ cat tp3.c
> int a;
> 
> void func(void)
> {
> 	*(volatile int *)&a = 10;
> 	*(volatile int *)&a = 20;
> }
> $ gcc -Os -S tp3.c
> $ cat tp3.s
> ...
> movl    $10, a
> movl    $20, a
> ...

I'm curious about one minor tangential point.  Why, instead of:

	b = *(volatile int *)&a;

why can't this just be expressed as:

	b = (volatile int)a;

Isn't it the contents of a that's volatile, i.e. it's value can change
invisibly to the compiler, and that's why you want to force a read from
memory?  Why do you need the "*(volatile int *)&" construct?

						-Bill
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