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

Mathieu Desnoyers
2008-11-07T21:32:27Z

* Steven Rostedt (rostedt@goodmis.org) wrote:
>
> On Fri, 7 Nov 2008, Mathieu Desnoyers wrote:
> >
> > I want to make sure
> >
> > __m_cnt_hi
> > is read before
> > mmio cnt_lo read
>
> Hmm, let me make sure I understand why there is no wmb.
>
> Paul, can you verify this?
>
> Mathieu, you do the following:
>
> read a
> smp_rmb
> reab b
> if (test b)
> write a
>
> So the idea is that you must read b to test it. And since we must read a
> before reading b we can see that we write a before either?
>
> The question remains, can the write happen before either of the reads?
>

write a cannot happen before read a (same variable).
write a must happen after read b because it depends on the b value. It
makes sure the the side-effect of "write a" is seen by other CPUs
*after* we have read the b value.

> But since the read b is reading the hw clock, perhaps that just implies a
> wmb on the hardware side?
>

It makes sense. The hardware clock has no cache coherency problem.. so
it could be seen as doing wmb() after each data update.

Mathieu

> -- Steve

--
Mathieu Desnoyers
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