Messages in this thread | | | Subject | Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb() | From | Peter Zijlstra <> | Date | Fri, 07 Nov 2008 20:32:02 +0100 |
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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?
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