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    SubjectRe: LDD3 pitfalls (was Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures)
    Stefan Richter wrote:
    > Nick Piggin wrote:
    >> Stefan Richter wrote:
    >>> Nick Piggin wrote:
    >>>
    >>>> I don't know why people would assume volatile of atomics. AFAIK, most
    >>>> of the documentation is pretty clear that all the atomic stuff can be
    >>>> reordered etc. except for those that modify and return a value.
    >>>
    >>> Which documentation is there?
    >> Documentation/atomic_ops.txt
    >>
    >>
    >>> For driver authors, there is LDD3. It doesn't specifically cover
    >>> effects of optimization on accesses to atomic_t.
    >>>
    >>> For architecture port authors, there is Documentation/atomic_ops.txt.
    >>> Driver authors also can learn something from that document, as it
    >>> indirectly documents the atomic_t and bitops APIs.
    >>>
    >> "Semantics and Behavior of Atomic and Bitmask Operations" is
    >> pretty direct :)
    >>
    >> Sure, it says that it's for arch maintainers, but there is no
    >> reason why users can't make use of it.
    >
    >
    > Note, LDD3 page 238 says: "It is worth noting that most of the other
    > kernel primitives dealing with synchronization, such as spinlock and
    > atomic_t operations, also function as memory barriers."
    >
    > I don't know about Linux 2.6.10 against which LDD3 was written, but
    > currently only _some_ atomic_t operations function as memory barriers.
    >
    > Besides, judging from some posts in this thread, saying that atomic_t
    > operations dealt with synchronization may not be entirely precise.

    atomic_t is often used as the basis for implementing more sophisticated
    synchronization mechanisms, such as rwlocks. Whether or not they are designed
    for that purpose, the atomic_* operations are de facto synchronization primitives.

    -- Chris
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