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SubjectRe: [PATCH 1/3] perf, x86: Fix accidentally ack'ing a second event on intel perf counter
On Thu, Sep 02, 2010 at 11:26:27PM +0400, Cyrill Gorcunov wrote:
> Lets assume 1 counters is triggered and global bit is set as well
>
> we have here
>
> status = intel_pmu_get_status();
>
> > perf_sample_data_init(&data, 0);
> >
> > @@ -728,6 +728,7 @@ static int intel_pmu_handle_irq(struct pt_regs *regs)
> >
> > loops = 0;
> > again:
> > + intel_pmu_ack_status(status);
>
> So here we write just being read value back to CTRL register and _if_ new
> overflow happened in this window we've cleared it without processing.

No, you have to write a '1' to clear. If a new bit is set after we read
it, then our 'status' var should have a '0' in that bit and thus will not
get cleared when we ack it.

Cheers,
Don


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