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

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Jerry Jiang wrote:

On Wed, 08 Aug 2007 02:47:53 -0400
Chris Snook <[email protected]> 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
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