Re: perf: fuzzer triggered warning in intel_pmu_drain_pebs_nhm()

[Peter Zijlstra]

On Fri, Jul 03, 2015 at 08:08:27PM +0000, Liang, Kan wrote:
> If we cleared the last bit, we not only drain the buffer but also decrease 
> the event->ctx->pmu, which is used to flush the PEBS buffer during
> context switches.
> We need to disable cpuc->pebs_enabled before changing 
> event->ctx->pmu as below.
> 

Indeed, mind sending a proper patch so I can press 'A' on it?

> diff --git a/arch/x86/kernel/cpu/perf_event_intel_ds.c 
> b/arch/x86/kernel/cpu/perf_event_intel_ds.c
> index 71fc402..76285c1 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel_ds.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c
> @@ -754,6 +754,11 @@ void intel_pmu_pebs_disable(struct 
> perf_event *event)
>         struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>         struct hw_perf_event *hwc = &event->hw;
>         struct debug_store *ds = cpuc->ds;
> +       bool large_pebs = ds->pebs_interrupt_threshold >
> +                         ds->pebs_buffer_base + x86_pmu.pebs_record_size;
> +
> +       if (large_pebs)
> +               intel_pmu_drain_pebs_buffer();
> 
>         cpuc->pebs_enabled &= ~(1ULL << hwc->idx);
> 
> @@ -762,12 +767,8 @@ void intel_pmu_pebs_disable(struct 
> perf_event *event)
>         else if (event->hw.flags & PERF_X86_EVENT_PEBS_ST)
>                 cpuc->pebs_enabled &= ~(1ULL << 63);
> 
> -       if (ds->pebs_interrupt_threshold >
> -           ds->pebs_buffer_base + x86_pmu.pebs_record_size) {
> -               intel_pmu_drain_pebs_buffer();
> -               if (!pebs_is_enabled(cpuc))
> -                       perf_sched_cb_dec(event->ctx->pmu);
> -       }
> +       if (large_pebs && !pebs_is_enabled(cpuc))
> +               perf_sched_cb_dec(event->ctx->pmu);
> 
>         if (cpuc->enabled)
>                 wrmsrl(MSR_IA32_PEBS_ENABLE, cpuc->pebs_enabled);
> 
> 
> 
> Thanks,
> Kan
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