Re: [PATCH v4 10/10] x86/mm: Try to preserve old TLB entries using PCID

[Andy Lutomirski]

On Wed, Jul 5, 2017 at 5:18 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, Jun 29, 2017 at 08:53:22AM -0700, Andy Lutomirski wrote:
>> @@ -104,18 +140,20 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
>>
>>               /* Resume remote flushes and then read tlb_gen. */
>>               cpumask_set_cpu(cpu, mm_cpumask(next));
>
> Barriers should have a comment... what is being ordered here against
> what?

How's this comment?

        /*
         * Resume remote flushes and then read tlb_gen.  We need to do
         * it in this order: any inc_mm_tlb_gen() caller that writes a
         * larger tlb_gen than we read here must see our cpu set in
         * mm_cpumask() so that it will know to flush us.  The barrier
         * here synchronizes with inc_mm_tlb_gen().
         */