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SubjectRe: [tip: perf/core] perf/x86: Reset the dirty counter to prevent the leak for an RDPMC task
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On 4/16/2021 12:45 PM, Peter Zijlstra wrote:
> On Fri, Apr 16, 2021 at 03:01:48PM -0000, tip-bot2 for Kan Liang wrote:
>> @@ -2331,6 +2367,9 @@ static void x86_pmu_event_unmapped(struct perf_event *event, struct mm_struct *m
>> if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
>> return;
>>
>> + if (x86_pmu.sched_task && event->hw.target)
>> + perf_sched_cb_dec(event->ctx->pmu);
>> +
>> if (atomic_dec_and_test(&mm->context.perf_rdpmc_allowed))
>> on_each_cpu_mask(mm_cpumask(mm), cr4_update_pce, NULL, 1);
>> }
>
> 'perf test' on a kernel with CONFIG_DEBUG_PREEMPT=y gives:
>
> [ 244.439538] BUG: using smp_processor_id() in preemptible [00000000] code: perf/1771

If it's a preemptible env, I think we should disable the interrupts and
preemption to protect the sched_cb_list.

Seems we don't need perf_ctx_lock() here. I don't think we touch the
area in NMI. I think disabling the interrupts should be good enough to
protect the cpuctx.

How about the below patch?

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index e34eb72..45630beed 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2333,6 +2333,8 @@ static void x86_pmu_clear_dirty_counters(void)

static void x86_pmu_event_mapped(struct perf_event *event, struct
mm_struct *mm)
{
+ unsigned long flags;
+
if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
return;

@@ -2341,8 +2343,10 @@ static void x86_pmu_event_mapped(struct
perf_event *event, struct mm_struct *mm)
* and clear the existing dirty counters.
*/
if (x86_pmu.sched_task && event->hw.target) {
+ local_irq_save(flags);
perf_sched_cb_inc(event->ctx->pmu);
x86_pmu_clear_dirty_counters();
+ local_irq_restore(flags);
}

/*
@@ -2363,12 +2367,16 @@ static void x86_pmu_event_mapped(struct
perf_event *event, struct mm_struct *mm)

static void x86_pmu_event_unmapped(struct perf_event *event, struct
mm_struct *mm)
{
+ unsigned long flags;

if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
return;

- if (x86_pmu.sched_task && event->hw.target)
+ if (x86_pmu.sched_task && event->hw.target) {
+ local_irq_save(flags);
perf_sched_cb_dec(event->ctx->pmu);
+ local_irq_restore(flags);
+ }

if (atomic_dec_and_test(&mm->context.perf_rdpmc_allowed))
on_each_cpu_mask(mm_cpumask(mm), cr4_update_pce, NULL, 1);
Thanks,
Kan

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