Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

Peter Zijlstra
2008-11-07T19:32:57Z

On Fri, 2008-11-07 at 14:18 -0500, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@goodmis.org) wrote:
> >
> > On Fri, 7 Nov 2008, Mathieu Desnoyers wrote:
> > >
> > > __m_cnt_hi
> > > is read before
> > > mmio cnt_lo read
> > >
> > > for the detailed reasons explained in my previous discussion with
> > > Nicolas here :
> > > http://lkml.org/lkml/2008/10/21/1
> > >
> > > I use smp_rmb() to do this on SMP systems (hrm, actually, a rmb() could
> > > be required so it works also on UP systems safely wrt interrupts).
> >
> > smp_rmb turns into a compiler barrier on UP and should prevent the below
> > description.
> >
>
> Ah, right, preserving program order on UP should be enough. smp_rmb()
> then.

I'm not quite sure I'm following here. Is this a global hardware clock
you're reading from multiple cpus, if so, are you sure smp_rmb() will
indeed be enough to sync the read?

(In which case the smp_wmb() is provided by the hardware increasing the
clock?)

If these are per-cpu clocks then even in the smp case we'd be good with
a plain barrier() because you'd only ever want to read your own cpu's
clock (and have a separate __m_cnt_hi per cpu).

Or am I totally missing out on something?