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    SubjectRe: [PATCH] make cancel_rearming_delayed_work() reliable
    On 05/15, Tejun Heo wrote:
    >
    > Oleg Nesterov wrote:
    >
    > > So, try_to_grab_pending() should check "VALID && pointers equal" atomically.
    > > We can't do "if (VALID && cwq == get_wq_data(work))". We should do something
    > > like this
    > >
    > > (((long)cwq) | VALID | PENDING) == atomic_long_read(&work->data)
    > >
    > > Yes? I need to think more about this.
    >
    > I don't think the test for PENDING should be atomic too. cwq pointer
    > and VALID is one package. PENDING lives its own life as a atomic bit
    > switch.

    Yes sure, it should not be atomic. But (VALID && !PENDING) == BUG, so we
    can't just "kill" PENDING form the check above.

    > > BTW, in _theory_, spinlock() is not a read barrier, yes?
    >
    > It actually is.
    >
    > > From Documentation/memory-barriers.txt
    > >
    > > Memory operations that occur before a LOCK operation may appear to happen
    > > after it completes.
    >
    > Which means that spin_lock() isn't a write barrier.

    yes, it is not very clear which "Memory operations" memory-barriers.txt
    describes.

    > lock is read
    > barrier, unlock is write barrier.

    (To avoid a possible confusion: I am not arguing, I am trying to understand,
    and please also note "in _theory_" above)

    Is it so? Shoudn't we document this if it is true?

    > Otherwise, locking doesn't make much
    > sense.

    Why? Could you please give a code example we have which relies on this?

    > If we're going the barrier way, I think we're better off with
    > explicit smp_wmb(). It's only barrier() on x86/64.

    Yes. But note that we don't have any reason to do set_wq_data() under
    cwq->lock (this is also true for wake_up(->more_work) btw), so it makes
    sense to do this change anyway. And "wmb + spin_lock" looks a bit strange,
    I _suspect_ spin_lock() means a full barrier on most platforms.

    Could you also look at
    http://marc.info/?t=116275561700001&r=1

    and, in particular,
    http://marc.info/?l=linux-kernel&m=116281136122456

    Thanks!

    Oleg.

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