Subject: Re: [PATCH] perf_events: fix validate_event bug
From: Peter Zijlstra <peterz@infradead.org>
To: eranian@gmail.com
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, paulus@samba.org,
       perfmon2-devel@lists.sf.net, eranian@google.com
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Date: Mon, 23 Nov 2009 14:45:53 +0100
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On Mon, 2009-11-23 at 14:34 +0100, stephane eranian wrote:

> > Won't this give very funny results for mixed pmu groups?
> >
> 
> What do you mean by 'mixed pmu groups'?

We currently have a number of struct pmu objects:

 perf_ops_generic
 perf_ops_cpu_clock
 perf_ops_task_clock

which are all software based PMUs, and one of:

 pmu        (arch/x86/kernel/cpu/perf_event.c)
 power_pmu  (arch/powerpc/kernel/perf_event.c)

To represent the hardware PMU.

Now say you mix software events and hardware events into a single group,
the loop in validate_group:

  list_for_each_entry(sibling, &leader->sibling_list, group_entry) {
	if (!validate_event(&fake_pmu, sibling))
			return -ENOSPC;
  }

could pass a !hardware event into validate_event(), which currently
ignores it because event->pmu won't be &pmu, however if you remove that
check, it'll try and call x86 routines on a software event, which is
bound to go funny.

Now Frederic is going to make things more interesting by representing HW
breakpoints as another HW PMU (the distinction between hw/sw pmu is in
scheduling, you can always schedule a software event).

This weakens the !is_software_event(), in that !software doesn't tell
you which hardware event it is -- something which needs mending in your
more complex x86 constraints scheduling patch.

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