linux-kernel - Re: [PATCH 1/3] perf, x86: Fix accidentally ack'ing a second event on intel perf counter

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Date:	Thu, 2 Sep 2010 16:00:21 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	mingo@...e.hu, peterz@...radead.org, robert.richter@....com,
	fweisbec@...il.com, linux-kernel@...r.kernel.org,
	ying.huang@...el.com, ming.m.lin@...el.com, yinghai@...nel.org,
	andi@...stfloor.org, eranian@...gle.com
Subject: Re: [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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