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

[Mario Limonciello <mario.limonciello@amd.com>]

On 10/24/2023 11:51, Ingo Molnar wrote:
> 
> * 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

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

For good measure x86_pmu_starting_cpu() too?