Re: [PATCH 6/24] make atomic_read() behave consistently on frv

Arnd Bergmann
2007-08-14T22:18:10Z

On Tuesday 14 August 2007, Paul E. McKenney wrote:
> > #define order(x) asm volatile("" : "+m" (x))
>
> There was something very similar discussed earlier in this thread,
> with quite a bit of debate as to exactly what the "m" flag should
> look like.  I suggested something similar named ACCESS_ONCE in the
> context of RCU (http://lkml.org/lkml/2007/7/11/664):
>
>         #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
>
> The nice thing about this is that it works for both loads and stores.
> Not clear that order() above does this -- I get compiler errors when
> I try something like "b = order(a)" or "order(a) = 1" using gcc 4.1.2.

Well, it serves a different purpose: While your ACCESS_ONCE() macro is
an lvalue, the order() macro is a statement that can be used in place
of the barrier() macro. order() is the most lightweight barrier as it
only enforces ordering on a single variable in the compiler, but does
not have any side-effects visible to other threads, like the cache
line access in ACCESS_ONCE has.