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

[Chris Snook <csnook@redhat.com>]

Jerry Jiang wrote:
> 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?

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

	-- Chris