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

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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 <[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

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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