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.
