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Date: Thu, 21 May 2015 14:56:15 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Stephane Eranian <eranian@google.com>
Cc: Ingo Molnar <mingo@kernel.org>, Vince Weaver <vincent.weaver@maine.edu>,
        Jiri Olsa <jolsa@redhat.com>, "Liang, Kan" <kan.liang@intel.com>,
        LKML <linux-kernel@vger.kernel.org>, Andrew Hunter <ahh@google.com>,
        Maria Dimakopoulou <maria.n.dimakopoulou@gmail.com>
Subject: Re: [PATCH 01/10] perf,x86: Fix event/group validation
Message-ID: <20150521125615.GO3644@twins.programming.kicks-ass.net>
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On Thu, May 21, 2015 at 05:35:02AM -0700, Stephane Eranian wrote:
> Peter,
> 
> On Thu, May 21, 2015 at 4:17 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > Commit 43b4578071c0 ("perf/x86: Reduce stack usage of
> > x86_schedule_events()") violated the rule that 'fake' scheduling; as
> > used for event/group validation; should not change the event state.
> >
> > This went mostly un-noticed because repeated calls of
> > x86_pmu::get_event_constraints() would give the same result. And
> > x86_pmu::put_event_constraints() would mostly not do anything.
> >
> > Things could still go wrong esp. for shared_regs, because
> > cpuc->is_fake can return a different constraint and then the
> > put_event_constraint will not match up.
> >
> I don't follow this here. What do you mean by 'match up'?

Ah, I wrote that Changelog for a prior patch; which by writing the
changelog I found faulty.

But I then forgot to update the Changelog.

I was under the impression put_event_constraints() would actually take
the constraint as an argument, and with the below example, it would not
do put on the same it would get.

> > Commit e979121b1b15 ("perf/x86/intel: Implement cross-HT corruption
> > bug workaround") made the situation much worse by actually setting the
> > event->hw.constraint value to NULL, so when validation and actual
> > scheduling interact we get NULL ptr derefs.
> >
> 
> But  x86_schedule_events() does reset the hw.constraint for each invocation:
> 
>            c = x86_pmu.get_event_constraints(cpuc, i, cpuc->event_list[i]);
>            hwc->constraint = c;

Yes, so if you have:

	validate_group()

		hwc->constraint = c;

	<context switch>

		c = hwc->constraint;

The second c might not be the first.
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