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Date: Wed, 8 Aug 2007 16:27:55 +0800
From: Jerry Jiang <wjiang@resilience.com>
To: Chris Snook <csnook@redhat.com>
Cc: Chris Friesen <cfriesen@nortel.com>, Zan Lynx <zlynx@acm.org>,
       "Robert P. J. Day" <rpjday@mindspring.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: why are some atomic_t's not volatile, while most are?
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On Wed, 08 Aug 2007 02:47:53 -0400
Chris Snook <csnook@redhat.com> wrote:

> Chris Friesen wrote:
> > Chris Snook wrote:
> > 
> >> This is not a problem, since indirect references will cause the CPU to 
> >> fetch the data from memory/cache anyway.
> > 
> > Isn't Zan's sample code (that shows the problem) already using indirect 
> > references?
> 
> Yeah, I misinterpreted his conclusion.  I thought about this for a 
> while, and realized that it's perfectly legal for the compiler to re-use 
> a value obtained from atomic_read.  All that matters is that the read 
> itself was atomic.  The use (or non-use) of the volatile keyword is 
> really more relevant to the other atomic operations.  If you want to 
> guarantee a re-read from memory, use barrier().  This, incidentally, 
> uses volatile under the hood.
> 


So for example, without volatile

int a = read_atomic(v);
int b = read_atomic(v);

the compiler will optimize it as b = a, 
But with volatile, it will be forced to fetch v's value from memory
again.

So, come back our initial question, 

include/asm-v850/atomic.h:typedef struct { int counter; } atomic_t;

Why is it right without volatile?

-- Jerry


> 	-- Chris