Re: [PATCH v2 1/1] mm: fix the theoretical compound_lock() vs prep_new_page() race

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jan 10, 2014 at 05:12:27PM +0100, Oleg Nesterov wrote:
> The recent "[PATCH v6 tip/core/locking 3/8] Documentation/memory-barriers.txt:
> Prohibit speculative writes" from Paul says:
> 
> 	No SMP architecture currently supporting Linux allows speculative writes,
> 
> 	...
> 
> 	+ACCESS_ONCE(), which preserves the ordering between
> 	+the load from variable 'a' and the store to variable 'b':
> 	+
> 	+       q = ACCESS_ONCE(a);
> 	+       if (q) {
> 	+               ACCESS_ONCE(b) = p;
> 	+               do_something();
> 	+       }
> 
> 
> We can't use ACCESS_ONCE(), but I think that
> 
> 		if (PageTail(page)) {
> 			barrier();
> 			compound_lock(page_head);
> 		}
> 
> should obviously work (even if compound_lock() didn't imply mb).

The compiler can actually screw you over if that's preceded by something
like: SetPageTail(page). In which case it can prove that if (PageTail())
is a non-condition.

But yes, barring that, the version with barrier() in should stop the
compiler from doing most terrible things and it ought to work out.