Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function_many())

Peter Zijlstra
2009-02-17T12:41:39Z

On Tue, 2009-02-17 at 10:29 +0100, Peter Zijlstra wrote:
> > It would be nice to have a comment which explains how can we miss the
> > first addition without read_barrier_depends(). And why only on alpha.
>
> Paul, care to once again enlighten us? The best I can remember is that
> alpha has split caches, and the rmb is needed for them to become
> coherent -- no other arch is crazy in exactly that way.
>
> But note that read_barrier_depends() is not quite a NOP for !alpha, it
> does that ACCESS_ONCE() thing, which very much makes a difference, even
> on x86.

I've been saying crazy stuff, read_barrier_depends() is a NOP, I got
confused with rcu_dereference(). My bad.