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

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Message-ID: <46C997B1.1010800@redhat.com>
Date:	Mon, 20 Aug 2007 09:31:29 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Christoph Lameter <clameter@....com>
CC:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Paul Mackerras <paulus@...ba.org>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	heiko.carstens@...ibm.com, horms@...ge.net.au,
	linux-kernel@...r.kernel.org, rpjday@...dspring.com, ak@...e.de,
	netdev@...r.kernel.org, cfriesen@...tel.com,
	akpm@...ux-foundation.org, jesper.juhl@...il.com,
	linux-arch@...r.kernel.org, zlynx@....org, satyam@...radead.org,
	schwidefsky@...ibm.com, davem@...emloft.net, wensong@...ux-vs.org,
	wjiang@...ilience.com
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
 architectures

Christoph Lameter wrote:
> On Fri, 17 Aug 2007, Paul E. McKenney wrote:
> 
>> On Sat, Aug 18, 2007 at 08:09:13AM +0800, Herbert Xu wrote:
>>> On Fri, Aug 17, 2007 at 04:59:12PM -0700, Paul E. McKenney wrote:
>>>> gcc bugzilla bug #33102, for whatever that ends up being worth.  ;-)
>>> I had totally forgotten that I'd already filed that bug more
>>> than six years ago until they just closed yours as a duplicate
>>> of mine :)
>>>
>>> Good luck in getting it fixed!
>> Well, just got done re-opening it for the third time.  And a local
>> gcc community member advised me not to give up too easily.  But I
>> must admit that I am impressed with the speed that it was identified
>> as duplicate.
>>
>> Should be entertaining!  ;-)
> 
> Right. ROTFL... volatile actually breaks atomic_t instead of making it 
> safe. x++ becomes a register load, increment and a register store. Without 
> volatile we can increment the memory directly. It seems that volatile 
> requires that the variable is loaded into a register first and then 
> operated upon. Understandable when you think about volatile being used to 
> access memory mapped I/O registers where a RMW operation could be 
> problematic.

So, if we want consistent behavior, we're pretty much screwed unless we use 
inline assembler everywhere?

	-- Chris
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