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

Jerry Jiang
2007-08-08T08:33:39Z

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