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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