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


    > -- Chris
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