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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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