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    SubjectRe: [PATCH] Document that wake_up(), complete() and co. imply a full memory barrier
    On 04/22, Ingo Molnar wrote:
    >
    > * Oleg Nesterov <oleg@redhat.com> wrote:
    >
    > > So, I think that try_to_wake_up() implies that the LOADS after it
    > > can't be reordered with STOREs before it (and wmb() of course).
    >
    > Note that the patch David sent says "full memory barrier", not "full
    > memory barrier wrt. task->state":
    >
    > + (*) wake_up(), try_to_wake_up() and co. imply a full memory barrier.
    > +
    > + (*) complete() and co. imply a full memory barrier.
    >
    > These statements are not true in that form, as this code does not
    > imply a full memory barrier. It does imply one on task->state
    > _alone_ (and a couple of other wq-internal variables it happens to
    > read for sure).
    >
    > But even that one isnt entirely true in the two sub-cases i noted:
    > the !wq case (which can happen in object state teardown) and the
    > special ->func handler (which can happen in custom wakeup code a'la
    > eventpoll).
    >
    > So adding a comment that says "this is a full memory barrier" is
    > simply not true to that extent, and is easily misunderstood. Adding
    > "this is a fully memory barrier for task->state dependent data flow"
    > would be more correct. (with a 'as long as wq is not NULL, and as
    > long as the code using this isnt overriding ->func)
    >
    > Agreed?

    Yes sure.

    Except... not that it really matters, but the reading of ->state is
    not "special". I mean,

    STORE;
    try_to_wake_up();
    LOAD;

    in this case try_to_wake_up() acts as a barrier for STORE/LOAD. But
    probably we should not rely on this. So personally I agree with
    "for task->state dependent data flow" above.

    Oleg.



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