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

Arnd Bergmann
2007-08-15T14:52:28Z

On Wednesday 15 August 2007, Paul E. McKenney wrote:

> ACCESS_ONCE() is indeed intended to be used when actually loading or
> storing the variable. That said, I must admit that it is not clear to me
> why you would want to add an extra order() rather than ACCESS_ONCE()ing
> one or both of the adjacent accesses to that same variable.
>
> So, what am I missing?

You're probably right, the only case I can construct is something like

if (ACCESS_ONCE(x)) {
...
ACCESS_ONCE(x)++;
}

which would be slightly less efficient than

if (x)
x++;
order(x);

because in the first case, you need to do two ordered accesses
but only one in the second case. However, I can't think of a case
where this actually makes a noticable difference in real life.