page
Unlocks the page and wakes up sleepers in ___wait_on_page_locked. Also wakes sleepers in wait_on_page_writeback because the wakeup mechananism between PageLocked pages and PageWriteback pages is shared. But that's OK - sleepers in wait_on_page_writeback just go back to sleep.
The mb is necessary to enforce ordering between the clear_bit and the read of the waitqueue (to avoid SMP races with a parallel wait_on_page_locked).