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

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Date:	Fri, 3 Sep 2010 00:36:08 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Don Zickus <dzickus@...hat.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 04:00:21PM -0400, Don Zickus wrote:
> 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.
> 

Yeah, messed {0;1} values, thanks!

> Cheers,
> Don
> 
	-- Cyrill
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