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