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
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