Re: [PATCH] Revert "perf/x86: Allow zero PEBS status with only single active event"

[Stephane Eranian]

On Tue, Mar 16, 2021 at 5:28 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>
>
>
> On 3/16/2021 3:22 AM, Namhyung Kim wrote:
> > Hi Peter and Kan,
> >
> > On Thu, Mar 4, 2021 at 5:22 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >>
> >> On Wed, Mar 03, 2021 at 02:53:00PM -0500, Liang, Kan wrote:
> >>> On 3/3/2021 1:59 PM, Peter Zijlstra wrote:
> >>>> On Wed, Mar 03, 2021 at 05:42:18AM -0800, kan.liang@xxxxxxxxxxxxxxx wrote:
> >>
> >>>>> +++ b/arch/x86/events/intel/ds.c
> >>>>> @@ -2000,18 +2000,6 @@ static void intel_pmu_drain_pebs_nhm(struct pt_regs *iregs, struct perf_sample_d
> >>>>>                            continue;
> >>>>>                    }
> >>>>> -         /*
> >>>>> -          * On some CPUs the PEBS status can be zero when PEBS is
> >>>>> -          * racing with clearing of GLOBAL_STATUS.
> >>>>> -          *
> >>>>> -          * Normally we would drop that record, but in the
> >>>>> -          * case when there is only a single active PEBS event
> >>>>> -          * we can assume it's for that event.
> >>>>> -          */
> >>>>> -         if (!pebs_status && cpuc->pebs_enabled &&
> >>>>> -                 !(cpuc->pebs_enabled & (cpuc->pebs_enabled-1)))
> >>>>> -                 pebs_status = cpuc->pebs_enabled;
> >>>>
> >>>> Wouldn't something like:
> >>>>
> >>>>                      pebs_status = p->status = cpus->pebs_enabled;
> >>>>
> >>>
> >>> I didn't consider it as a potential solution in this patch because I don't
> >>> think it's a proper way that SW modifies the buffer, which is supposed to be
> >>> manipulated by the HW.
> >>
> >> Right, but then HW was supposed to write sane values and it doesn't do
> >> that either ;-)
> >>
> >>> It's just a personal preference. I don't see any issue here. We may try it.
> >>
> >> So I mostly agree with you, but I think it's a shame to unsupport such
> >> chips, HSW is still a plenty useable chip today.
> >
> > I got a similar issue on ivybridge machines which caused kernel crash.
> > My case it's related to the branch stack with PEBS events but I think
> > it's the same issue.  And I can confirm that the above approach of
> > updating p->status fixed the problem.
> >
> > I've talked to Stephane about this, and he wants to make it more
> > robust when we see stale (or invalid) PEBS records.  I'll send the
> > patch soon.
> >
>
> Hi Namhyung,
>
> In case you didn't see it, I've already submitted a patch to fix the
> issue last Friday.
> https://lore.kernel.org/lkml/1615555298-140216-1-git-send-email-kan.liang@xxxxxxxxxxxxxxx/
> But if you have a more robust proposal, please feel free to submit it.
>
This fixes the problem on the older systems. The other problem we
identified related to the
PEBS sample processing code is that you can end up with uninitialized
perf_sample_data
struct passed to perf_event_overflow():

 setup_pebs_fixed_sample_data(pebs, data)
{
        if (!pebs)
                return;
        perf_sample_data_init(data);  <<< must be moved before the if (!pebs)
        ...
}

__intel_pmu_pebs_event(pebs, data)
{
        setup_sample(pebs, data)
        perf_event_overflow(data);
        ...
}

If there is any other reason to get a pebs = NULL in fix_sample_data()
or adaptive_sample_data(), then
you must call perf_sample_data_init(data) BEFORE you return otherwise
you end up in perf_event_overflow()
with uninitialized data and you may die as follows:

[<ffffffff812f283d>] ? perf_output_copy+0x4d/0xb0
[<ffffffff812e0fb1>] perf_output_sample+0x561/0xab0
[<ffffffff812e0952>] ? __perf_event_header__init_id+0x112/0x130
[<ffffffff812e1be1>] ? perf_prepare_sample+0x1b1/0x730
[<ffffffff812e21b9>] perf_event_output_forward+0x59/0x80
[<ffffffff812e0634>] ? perf_event_update_userpage+0xf4/0x110
[<ffffffff812e4468>] perf_event_overflow+0x88/0xe0
[<ffffffff810175b8>] __intel_pmu_pebs_event+0x328/0x380

This all stems from get_next_pebs_record_by_bit()  potentially
returning NULL but the NULL is not handled correctly
by the callers. This is what I'd like to see cleaned up in
__intel_pmu_pebs_event() to  avoid future problems.

I have a patch that moves the perf_sample_data_init() and I can send
it to LKML but it would also need the cleanup
for get_next_pebs_record_by_bit() to be complete.

Thanks.