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    SubjectRe: [RFC PATCH] mm: silence soft lockups from unlock_page
    On 07/23, Linus Torvalds wrote:
    >
    > So here's a v2, now as a "real" commit with a commit message and everything.

    I am already sleeping, will read it tomorrow, but at first glance...

    > @@ -1013,18 +1014,40 @@ static int wake_page_function(wait_queue_entry_t *wait, unsigned mode, int sync,
    > if (wait_page->bit_nr != key->bit_nr)
    > return 0;
    >
    > + /* Stop walking if it's locked */
    > + if (wait->flags & WQ_FLAG_EXCLUSIVE) {
    > + if (test_and_set_bit(key->bit_nr, &key->page->flags))
    > + return -1;
    > + } else {
    > + if (test_bit(key->bit_nr, &key->page->flags))
    > + return -1;
    > + }
    > +
    > /*
    > - * Stop walking if it's locked.
    > - * Is this safe if put_and_wait_on_page_locked() is in use?
    > - * Yes: the waker must hold a reference to this page, and if PG_locked
    > - * has now already been set by another task, that task must also hold
    > - * a reference to the *same usage* of this page; so there is no need
    > - * to walk on to wake even the put_and_wait_on_page_locked() callers.
    > + * Let the waiter know we have done the page flag
    > + * handling for it (and the return value lets the
    > + * wakeup logic count exclusive wakeup events).
    > */
    > - if (test_bit(key->bit_nr, &key->page->flags))
    > - return -1;
    > + ret = (wait->flags & WQ_FLAG_EXCLUSIVE) != 0;
    > + wait->flags |= WQ_FLAG_WOKEN;
    > + wake_up_state(wait->private, mode);
    >
    > - return autoremove_wake_function(wait, mode, sync, key);
    > + /*
    > + * Ok, we have successfully done what we're waiting for,
    > + * and we can unconditionally remove the wait entry.
    > + *
    > + * Note that this has to be the absolute last thing we do,
    > + * since after list_del_init(&wait->entry) the wait entry
    > + * might be de-allocated and the process might even have
    > + * exited.
    > + *
    > + * We _really_ should have a "list_del_init_careful()" to
    > + * properly pair with the unlocked "list_empty_careful()"
    > + * in finish_wait().
    > + */
    > + smp_mb();
    > + list_del_init(&wait->entry);

    I think smp_wmb() would be enough, but this is minor.

    We need a barrier between "wait->flags |= WQ_FLAG_WOKEN" and list_del_init(),

    But afaics we need another barrier, rmb(), in wait_on_page_bit_common() for
    the case when wait->private was not blocked; we need to ensure that if
    finish_wait() sees list_empty_careful() == T then we can't miss WQ_FLAG_WOKEN.

    Oleg.

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