__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
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