Re: [PATCH HACK RFC] cpu: Prevent late-arriving interrupts from disrupting offline

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 6/3/19 1:44 PM, Mark Rutland wrote:
> On Mon, Jun 03, 2019 at 10:38:48AM +0200, Peter Zijlstra wrote:
>> On Sat, Jun 01, 2019 at 06:12:53PM -0700, Paul E. McKenney wrote:
>>> Scheduling-clock interrupts can arrive late in the CPU-offline process,
>>> after idle entry and the subsequent call to cpuhp_report_idle_dead().
>>> Once execution passes the call to rcu_report_dead(), RCU is ignoring
>>> the CPU, which results in lockdep complaints when the interrupt handler
>>> uses RCU:
>>
>>> diff --git a/kernel/cpu.c b/kernel/cpu.c
>>> index 448efc06bb2d..3b33d83b793d 100644
>>> --- a/kernel/cpu.c
>>> +++ b/kernel/cpu.c
>>> @@ -930,6 +930,7 @@ void cpuhp_report_idle_dead(void)
>>>   	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
>>>   
>>>   	BUG_ON(st->state != CPUHP_AP_OFFLINE);
>>> +	local_irq_disable();
>>>   	rcu_report_dead(smp_processor_id());
>>>   	st->state = CPUHP_AP_IDLE_DEAD;
>>>   	udelay(1000);
>>
>> Urgh... I'd almost suggest we do something like the below.
>>
>>
>> But then I started looking at the various arch_cpu_idle_dead()
>> implementations and ran into arm's implementation, which is calling
>> complete() where generic code already established this isn't possible
>> (see for example cpuhp_report_idle_dead()).
> 
> IIRC, that should have been migrated over to cpu_report_death(), as
> arm64 was in commit:
> 
>    05981277a4de1ad6 ("arm64: Use common outgoing-CPU-notification code")
> 
> ... but it looks like Paul's patch to do so [1] fell through the cracks;
> I'm not aware of any reason that shouldn't have been taken.
>    
> [1] https://lore.kernel.org/lkml/1431467407-1223-8-git-send-email-paulmck@linux.vnet.ibm.com/
> 
> Paul, do you want to resend that?

Please do. We're carrying this patch out-of-tree for while now in our 
EAS integration to get cpu hotplug tests passing on TC2 (arm).

-- Dietmar