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    Subject__down() fails to provide with acquisition semantics
    I have got a concern about "__down()".
    Can someone explain me, please, how it is assumed to work?

    Apparently, "__down()" fails to provide with acquisition semantics
    in certain situations.

    Let's assume the semaphore is unavailable and there is nobody
    waiting for it.

    The next requester enters the slow path.

    Let's assume the owner of the semaphore releases it just before
    the requester would execute the line:

    if (!atomic_add_negative(sleepers - 1, &sem->count)) {

    Therefore the loop gets broken, the requester returns without
    going to sleep.

    ia64_fetchadd(i, v, rel)

    At least on ia64, "atomic_add_negative()" provides with release

    "remove_wait_queue_locked()" does not care for acquisition semantics.
    "wake_up_locked()" finds an empty list, it does nothing.
    "spin_unlock_irqrestore()" does release semantics.

    The requester is granted the semaphore and s/he enters the critical
    section without making sure that the memory accesses s/he wants to
    issue in the critical section cannot be made globally visible before
    "atomic_add_negative()" is.

    We need an acquisition semantics (at least) before entering any
    critical section. Should not we have something like:

    if (atomic_add_acq(sleepers - 1, &sem->count) /* ret: new val */ >= 0){

    "atomic_add_acq()" would provide with acquisition semantics in
    an architecture dependent way.
    I think it should be made more explicit that this routine should
    provide with the architecture dependent memory fencing.

    What is the use of the "wake_up_locked(&sem->wait)"?
    The other requester woke up, will find the semaphore unavailable...

    Another question: is there any reason keeping an ia64 version when
    lib/semaphore-sleepers.c and arch/ia64/kernel/semaphore.c do not
    really differ?


    Zoltan Menyhart
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      Last update: 2007-09-14 12:49    [W:0.021 / U:49.892 seconds]
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