Messages in this thread | | | Subject | Re: [PATCH 2/9] perf_counter: fix update_userpage() | From | Peter Zijlstra <> | Date | Thu, 02 Apr 2009 12:36:27 +0200 |
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On Thu, 2009-04-02 at 20:58 +1100, Paul Mackerras wrote: > Peter Zijlstra writes: > > > > Good point. This should work, though: > > > > > > do { > > > seq = pc->lock; > > > barrier(); > > > value = read_pmc(pc->index) + pc->offset; > > > barrier(); > > > } while (pc->lock != seq); > > > return value; > > > > I don't think you need the first barrier(), all you need to avoid is it > > reusing the first pc->lock read, so one should suffice. > > I need it to make sure that the compiler doesn't put the load of > pc->index or pc->offset before the first load of pc->lock. The second > barrier is needed to make sure the compiler puts the second load of > pc->lock after the loads of pc->index and pc->offset. So I think I do > need to barrier()s (but only compiler barriers, not cpu memory > barriers).
Ah, you're right indeed.
> > Also, you need to handle the !pc->index case. > > Hmmm, yeah. I claim that read_pmc(0) always returns 0. :)
Hehe :-)
Ok, updated that patch.
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