Re: thp and memory barrier assumptions

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2012-07-26 at 22:31 +0200, Peter Zijlstra wrote:
> __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
> 


Also, what is that barrier() in handle_mm_fault() doing? And why doesn't
it have a comment explaining that?