Messages in this thread | | | Date | Wed, 8 Aug 2007 16:27:55 +0800 | From | Jerry Jiang <> | Subject | Re: why are some atomic_t's not volatile, while most are? |
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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
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