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

[Ingo Molnar <mingo@kernel.org>]

* Ingo Molnar <mingo@kernel.org> wrote:

> 
> * Mario Limonciello <mario.limonciello@amd.com> 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@vger.kernel.org # 6.1+
> > Fixes: ca5b7c0d9621 ("perf/x86/amd/lbr: Add LbrExtV2 branch record support")
> > Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
> > ---
> >  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