netdev - Re: [PATCH] make atomic_t volatile on all architectures

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Date:	Thu, 09 Aug 2007 19:57:55 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Chuck Ebbert <cebbert@...hat.com>
Cc:	Chris Snook <csnook@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, ak@...e.de, heiko.carstens@...ibm.com,
	davem@...emloft.net, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, wensong@...ux-vs.org, horms@...ge.net.au,
	wjiang@...ilience.com, cfriesen@...tel.com, zlynx@....org
Subject: Re: [PATCH] make atomic_t volatile on all architectures

On Thu, 2007-08-09 at 13:36 -0400, Chuck Ebbert wrote:
> > Fair enough.  Casting to (volatile int *) will give us the behavior
> > people expect when using atomic_t without needing to use inefficient
> > barriers.
> > 
> 
> You can use this forget() macro to make the compiler reread a
> variable:
> 
> #define forget(var) asm volatile ("" : "=m"(var))

You need to specify a "m"(var) input constraint as well. Without it the
compiler might remove the initialization of var. E.g.

void fn(void)
{
	int var = 0;
	forget(var);
	/* now var can have any value. */
}

-- 
blue skies,
  Martin.

"Reality continues to ruin my life." - Calvin.


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