Re: [PATCH 2/2] perf/x86/amd: Don't allow pre-emption in amd_pmu_lbr_reset()

[Mario Limonciello]

On 10/24/2023 11:51, Ingo Molnar wrote:
> * Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> > * Mario Limonciello <mario.limonciello@xxxxxxx> wrote:
> >
> > > Fixes a BUG reported during suspend to ram testing.
> > >
> > > ```
> > > [  478.274752] BUG: using smp_processor_id() in preemptible [00000000] code: rtcwake/2948
> > > [  478.274754] caller is amd_pmu_lbr_reset+0x19/0xc0
> > > ```
> > >
> > > Cc: stable@xxxxxxxxxxxxxxx # 6.1+
> > > Fixes: ca5b7c0d9621 ("perf/x86/amd/lbr: Add LbrExtV2 branch record support")
> > > Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx>
> > > ---
> > >   arch/x86/events/amd/lbr.c | 3 ++-
> > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/events/amd/lbr.c b/arch/x86/events/amd/lbr.c
> > > index eb31f850841a..5b98e8c7d8b7 100644
> > > --- a/arch/x86/events/amd/lbr.c
> > > +++ b/arch/x86/events/amd/lbr.c
> > > @@ -321,7 +321,7 @@ int amd_pmu_lbr_hw_config(struct perf_event *event)
> > >   
> > >   void amd_pmu_lbr_reset(void)
> > >   {
> > > -	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> > > +	struct cpu_hw_events *cpuc = get_cpu_ptr(&cpu_hw_events);
> > >   	int i;
> > >   
> > >   	if (!x86_pmu.lbr_nr)
> > > @@ -335,6 +335,7 @@ void amd_pmu_lbr_reset(void)
> > >   
> > >   	cpuc->last_task_ctx = NULL;
> > >   	cpuc->last_log_id = 0;
> > > +	put_cpu_ptr(&cpu_hw_events);
> > >   	wrmsrl(MSR_AMD64_LBR_SELECT, 0);
> > >   }
> >
> > Weird, amd_pmu_lbr_reset() is called from these places:
> >
> >    - amd_pmu_lbr_sched_task(): during task sched-in during
> >      context-switching, this should already have preemption disabled.
> >
> >    - amd_pmu_lbr_add(): this gets indirectly called by amd_pmu::add
> >      (amd_pmu_add_event()), called by event_sched_in(), which too should have
> >      preemption disabled.
> >
> > I clearly must have missed some additional place it gets called in.
>
> Just for completeness, the additional place I missed is
> amd_pmu_cpu_reset():
>
>                  static_call(amd_pmu_branch_reset)();
>
> ... and the amd_pmu_branch_reset static call is set up with
> amd_pmu_lbr_reset, which is why git grep missed it.
>
> Anyway, amd_pmu_cpu_reset() is very much something that should run
> non-preemptable to begin with, so your patch only papers over the real
> problem AFAICS.
>
> Thanks,
>
> 	Ingo

In that case - should preemption be disabled for all of 
x86_pmu_dying_cpu() perhaps?

For good measure x86_pmu_starting_cpu() too?