linux-kernel - Re: [PATCH 4/4] [x86] perf: fix accidentally ack'ing a second event on intel perf counter

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Message-ID: <20100902154743.GI4879@redhat.com>
Date:	Thu, 2 Sep 2010 11:47:43 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Robert Richter <robert.richter@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 4/4] [x86] perf: fix accidentally ack'ing a second event
 on intel perf counter

On Thu, Sep 02, 2010 at 04:39:02PM +0200, Stephane Eranian wrote:
> Don,
> 
> On Thu, Sep 2, 2010 at 4:19 PM, Don Zickus <dzickus@...hat.com> wrote:
> > On Thu, Sep 02, 2010 at 10:13:19AM +0200, Stephane Eranian wrote:
> >> Robert,
> >>
> >> Do you have the test program you used to test this?
> >> I believe the NHM hack does not solve the problem, it
> >> just makes it harder to appear.
> >
> > Could be.
> >
> >>
> >> I suspect the real issue is that the GLOBAL_STATUS
> >> bitmask cannot be trusted. I'd like to verify this.
> >>
> >> Has the problem appear only on Nehalem or also on
> >> Westmere?
> >
> > I was able to duplicate on
> >
> > Intel(R) Core(TM) i5 CPU         650  @ 3.20GHz
> > Intel(R) Xeon(R) CPU           X5560  @ 2.80GHz
> >
> I managed to reproduce on core i7 860 (without patch4).
> Looking at the code again, I am dubious you ever execute
> the retry goto. If the PMU is disabled and you've just
> cleared the OVF_STAT, then I don't see where the new
> overflows would come from. But that's a separate problem.

I agree with you, but a printk before the goto proved otherwise! :-)

And a printk of the status bit that triggered the goto happened to be the
same one that we initially cleared.  Like I said when I initially posted
the patch, I am not sure why it works but it does do something to stem the
NMI.

There is probably a deeper problem here, I was just trying to get the
external/unknown nmis working again.

Cheers,
Don
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