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

[Chris Snook <csnook@redhat.com>]

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@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
> 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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