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    SubjectRe: [PATCH] It may not be assumed that wake_up(), finish_wait() and co. imply a memory barrier
    On 04/23, David Howells wrote:
    >
    > + complete();
    > + try_to_wake_up();
    > + wake_up();
    > + wake_up_all();
    > + wake_up_bit();
    > + wake_up_interruptible();
    > + wake_up_interruptible_all();
    > + wake_up_interruptible_nr();
    > + wake_up_interruptible_poll();
    > + wake_up_interruptible_sync();
    > + wake_up_interruptible_sync_poll();
    > + wake_up_locked();
    > + wake_up_locked_poll();
    > + wake_up_nr();
    > + wake_up_poll();
    > +
    > +After waking, and assuming it doesn't take a matching lock, the sleeper may
    > +need to interpolate a read or full memory barrier before accessing that state
    > +as finish_wait() does not imply a barrier either, and schedule() only implies a
    > +barrier on entry.

    Well. I am starting to suspect I missed something, but I disagree. Or I just
    (this is very possible) misunderstand the above.

    finish_wait() doesn't imply a barrier, but why this matters?

    And if we don't use prepare_to_wait() and just do

    for (;;) {
    set_current_state(WHATEVER);
    if (!CONDITION)
    schedule();
    break;
    }

    we do have mb(), but

    > + *
    > + * It should not be assumed that this function implies any sort of memory
    > + * barrier.
    > */
    > static int try_to_wake_up(struct task_struct *p, unsigned int state, int sync)

    unless try_to_wake_up() has a barrier semantics too, this code

    CONDITION = 1;
    wake_up_process(waiter);

    is not right, and that mb above can't help.


    Could you please give the code example which shows we need a barrier
    after finish_wait() ?

    I am just trying to understand what I missed.

    Oleg.



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