On Fri, Jun 03, 2016 at 02:23:10PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 03, 2016 at 05:08:27AM -0700, Paul E. McKenney wrote:
> > On Fri, Jun 03, 2016 at 11:38:34AM +0200, Peter Zijlstra wrote:
> > > On Fri, Jun 03, 2016 at 02:48:38PM +0530, Vineet Gupta wrote:
> > > > On Wednesday 25 May 2016 09:27 PM, Paul E. McKenney wrote:
> > > > > For your example, but keeping the compiler in check:
> > > > >
> > > > > if (READ_ONCE(a))
> > > > > WRITE_ONCE(b, 1);
> > > > > smp_rmb();
> > > > > WRITE_ONCE(c, 2);
> > >
> > > So I think it example is broken. The store to @c is not in fact
> > > dependent on the condition of @a.
> >
> > At first glance, the compiler could pull the write to "c" above the
> > conditional, but the "memory" constraint in smp_rmb() prevents this.
> > From a hardware viewpoint, the write to "c" does depend on the "if",
> > as the conditional branch does precede that write in execution order.
> >
> > But yes, this is using smp_rmb() in a very strange way, if that is
> > what you are getting at.
>
> Well, the CPU could decide that the store to C happens either way around
> the branch. I'm not sure I'd rely on CPUs not being _that_ clever.
If I remember correctly, both Power and ARM guarantee that the CPU won't
be that clever. Not sure about Itanium.
> test %a, $0
> jnz 1f
> mov $1, %b
> 1: mov $2, %c
>
> Its not too much to ask the CPU to look ahead 2 instructions to figure
> out the store into c is going to happen unconditionally.
>
> I would really only rely on stores immediately dependent on the
> conditional.
Given compiler tricks, a good choice.
Thanx, Paul
