Re: [PATCH] mm: prevent mmap_cache race in find_vma()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Apr 03, 2013 at 09:33:48AM -0700, Paul E. McKenney wrote:
> On Wed, Apr 03, 2013 at 06:45:51AM -0700, Ian Lance Taylor wrote:

[ . . . ]

> > If using a sufficiently recent version of GCC, you can get the
> > behaviour that I think you want by using
> >     __atomic_load(&x, __ATOMIC_RELAXED)
> 
> If this maps to the memory_order_relaxed token defined in earlier versions
> of the C11 standard, then this absolutely does -not-, repeat -not-, work
> for ACCESS_ONCE().  The relaxed load instead guarantees is that the load
> will be atomic with respect to other atomic stores to that same variable,
> in other words, it will prevent "load tearing" and "store tearing".  I
> also believe that it prevents reloading ...

In addition, even if the semantics of relaxed loads now guarantee against
load combining, note that ACCESS_ONCE() is also used to prevent combining
and splitting of stores, for example:

	ACCESS_ONCE(p) = give_me_a_pointer();

							Thanx, Paul

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