Re: [PATCH v2 05/10] x86/mm: Rework lazy TLB mode and TLB freshness tracking

[Andy Lutomirski]

On Wed, Jun 14, 2017 at 3:33 PM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> On 06/13/2017 09:56 PM, Andy Lutomirski wrote:
>> -     if (cpumask_test_cpu(cpu, &batch->cpumask))
>> +     if (cpumask_test_cpu(cpu, &batch->cpumask)) {
>> +             local_irq_disable();
>>               flush_tlb_func_local(&info, TLB_LOCAL_SHOOTDOWN);
>> +             local_irq_enable();
>> +     }
>> +
>
> Could you talk a little about why this needs to be local_irq_disable()
> and not preempt_disable()?  Is it about the case where somebody is
> trying to call flush_tlb_func_*() from an interrupt handler?

It's to prevent flush_tlb_func_local() and flush_tlb_func_remote()
from being run concurrently, which would cause flush_tlb_func_common()
to be reentered.  Either we'd need to be very careful in
flush_tlb_func_common() to avoid races if this happened, or we could
just disable interrupts around flush_tlb_func_local().  The latter is
fast and easy.

--Andy