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?
Date: Wed, 8 Aug 2007 16:27:55 +0800 [thread overview]
Message-ID: <20070808162755.61f50fbf.wjiang@resilience.com> (raw)
In-Reply-To: <46B96719.3050006@redhat.com>
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
next prev parent reply other threads:[~2007-08-08 8:33 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-07-01 12:49 why are some atomic_t's not volatile, while most are? Robert P. J. Day
2007-08-06 4:35 ` Jerry Jiang
2007-08-06 14:12 ` Chris Snook
2007-08-07 15:51 ` Chris Friesen
2007-08-07 20:32 ` Chris Snook
2007-08-07 21:02 ` Chris Friesen
2007-08-07 21:19 ` Chris Snook
2007-08-07 21:38 ` Chris Friesen
2007-08-07 22:02 ` Chris Snook
2007-08-07 22:46 ` Chris Friesen
2007-08-07 22:06 ` Jan Engelhardt
2007-08-07 22:49 ` Chris Friesen
2007-08-07 22:32 ` Zan Lynx
2007-08-08 1:31 ` Chris Snook
2007-08-08 4:50 ` Chris Friesen
2007-08-08 6:47 ` Chris Snook
2007-08-08 8:16 ` Jerry Jiang
2007-08-08 8:27 ` Jerry Jiang [this message]
2007-08-08 20:54 ` Chris Snook
2007-08-09 12:37 ` Robert P. J. Day
2007-08-09 12:52 ` Chris Snook
2007-08-09 18:02 ` Robert P. J. Day
2007-08-09 18:04 ` Robert P. J. Day
2007-08-08 2:27 ` Jerry Jiang
2007-08-08 5:39 ` Chris Snook
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