linux-kernel - Re: [PATCH v2 01/11] perf,x86: Fix event/group validation

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Message-ID: <CABPqkBQcKLoHCS3=_1cj4-JCTiOzWDc1=vGp85T-nnoAJQd5-Q@mail.gmail.com>
Date:	Tue, 26 May 2015 04:46:07 -0700
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	Jiri Olsa <jolsa@...hat.com>,
	"Liang, Kan" <kan.liang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Hunter <ahh@...gle.com>,
	Maria Dimakopoulou <maria.n.dimakopoulou@...il.com>
Subject: Re: [PATCH v2 01/11] perf,x86: Fix event/group validation

On Tue, May 26, 2015 at 3:12 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, May 26, 2015 at 02:24:38AM -0700, Stephane Eranian wrote:
>> On Fri, May 22, 2015 at 6:40 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Fri, May 22, 2015 at 03:29:06PM +0200, Peter Zijlstra wrote:
>> >> @@ -788,9 +788,9 @@ int x86_schedule_events(struct cpu_hw_ev
>> >>               x86_pmu.start_scheduling(cpuc);
>> >>
>> >>       for (i = 0, wmin = X86_PMC_IDX_MAX, wmax = 0; i < n; i++) {
>> >> +             cpuc->event_constraint[i] = NULL;
>> >
>> > ^^^ that is new, which is esp. important in light of the
>> > intel_get_event_constraints() hunk below, which would happily continue
>> > life with a garbage constraint.
>> >
>> You've moved the constraint list from event to cpuc. Yet, it is still
>> an array of pointers
>> to constraints. So here you are saying, that in the case validate_group() is
>> preempted and there is a context switch, there is still a risk of
>> overwriting the
>> constraint? I don't see how because validate_group() is using a fake_cpuc.
>> So yes, the cpuc->event_constraint[] array is modified but it is not the same
>> as the actual cpuc used by non-validate code. Or am I still missing something?
>>
>> When using dynamic constraints, we already have constraint storage in cpuc
>> (to avoid calling kmalloc() in ctxsw context). Thus, I am wondering if it would
>> not be easier to always use cpuc for constraint storage (no more pointers).
>
> No; the problem here is repeated use of the cpuc (the real one). Say one
> scheduling run installs a constraint pointer for event i. Then event i
> gets removed and another installed in the same spot.
>
> Then the next scheduling run will pick up the old pointer in
> intel_get_event_constraints() as a base for the new one.
>
But where is the code that says: skip reinstalling the constraint
in intel_get_event_constraints() because there is already a (stale)
one? I don't see where that is.
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