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Date: Thu, 21 May 2015 15:09:52 +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: <20150521130952.GQ3644@twins.programming.kicks-ass.net>
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On Thu, May 21, 2015 at 06:07:20AM -0700, Stephane Eranian wrote:
> On Thu, May 21, 2015 at 5:56 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Thu, May 21, 2015 at 05:35:02AM -0700, Stephane Eranian wrote:
> >> > 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;
> >
> Ok, you get that because validate_group() invokes x6_schedule_events() but
> on the fake_cpuc. This on fake_cpuc->event_list[]->hwc.
> 
> >         <context switch>
> >
> >                 c = hwc->constraint;
> >
> > The second c might not be the first.
> And where does this assignment come from?

That's a read. The <context switch> can include a call to
x86_schedule_events().

> For actual scheduling, we are using the actual cpuc, not fake_cpuc.
> Validate_group() does not modify global cpuc state. Or am I missing
> something?

No, but x86_schedule_event() can modify event state, which is the fail.


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