thp and memory barrier assumptions

[Peter Zijlstra <peterz@infradead.org>]

__do_huge_pmd_anonymous_page() contains:

                /*
                 * The spinlocking to take the lru_lock inside
                 * page_add_new_anon_rmap() acts as a full memory
                 * barrier to be sure clear_huge_page writes become
                 * visible after the set_pmd_at() write.
                 */
                page_add_new_anon_rmap(page, vma, haddr);


page_add_new_anon_rmap() doesn't look to actually do a LOCK+UNLOCK
except for unevictable pages.

But even if it did do an unconditional LOCK+UNLOCK that doesn't make a
full memory barrier, see Documentation/memory-barriers.txt.

In particular:

        *A = a;
        LOCK
        UNLOCK
        *B = b;

may occur as:

        LOCK, STORE *B, STORE *A, UNLOCK