linux-kernel - Re: why are some atomic_t's not volatile, while most are?

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Message-ID: <46BB0E1F.2030003@redhat.com>
Date:	Thu, 09 Aug 2007 08:52:47 -0400
From:	Chris Snook <csnook@...hat.com>
To:	"Robert P. J. Day" <rpjday@...dspring.com>
CC:	Jerry Jiang <wjiang@...ilience.com>,
	Chris Friesen <cfriesen@...tel.com>, Zan Lynx <zlynx@....org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: why are some atomic_t's not volatile, while most are?

Robert P. J. Day wrote:
> On Wed, 8 Aug 2007, Chris Snook wrote:
> 
>> Jerry Jiang wrote:
>>> 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?
>> Because atomic_t doesn't promise a memory fetch every time.  It merely
>> promises that any atomic_* operations will, in fact, be atomic.  For example,
>> posted today:
>>
>> http://lkml.org/lkml/2007/8/8/122
> 
> i'm sure that, when this is all done, i'll finally have an answer to
> my original question, "why are some atomic_t's not volatile, while
> most are?"
> 
> i'm almost scared to ask any more questions.  :-)
> 
> rday

Momentarily I'll be posting a patchset that makes all atomic_t and atomic64_t 
declarations non-volatile, and casts them to volatile inside of atomic[64]_read. 
  This will ensure consistent behavior across all architectures, and is in 
keeping with the philosophy that memory reads should be enforced in running 
code, not declarations.

I hope you don't mind that we're mooting the question by making the code more 
sensible.

	-- Chris
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