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Date: Thu, 21 May 2015 06:07:20 -0700
Message-ID: <CABPqkBTFPRwHX8zKERYFrMBVQor5GJcDd4ZcKt8WL-tjbJdzLw@mail.gmail.com>
Subject: Re: [PATCH 01/10] perf,x86: Fix event/group validation
From: Stephane Eranian <eranian@google.com>
To: Peter Zijlstra <peterz@infradead.org>
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>
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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?
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?
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