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

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Date:	Thu, 09 Aug 2007 20:20:46 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Chuck Ebbert <cebbert@...hat.com>, Chris Snook <csnook@...hat.com>,
	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 10:55 -0700, Linus Torvalds wrote:
> > You can use this forget() macro to make the compiler reread a variable:
> > 
> > #define forget(var) asm volatile ("" : "=m"(var))
> 
> No. That will also make the compiler "forget" any previous writes to it, 
> so it changes behaviour.
> 
> You'd have to use "+m".

Yes, though I would use "=m" on the output list and "m" on the input
list. The reason is that I've seen gcc fall on its face with an ICE on
s390 due to "+m". The explanation I've got from our compiler people was
quite esoteric, as far as I remember gcc splits "+m" to an input operand
and an output operand. Now it can happen that the compiler chooses two
different registers to access the same memory location. "+m" requires
that the two memory references are identical which causes the ICE if
they are not. I do not know if the current compilers still do this. Has
anyone else seen this happen ?

-- 
blue skies,
  Martin.

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

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