Re: [PATCH] Linux VM workaround for Knights Landing A/D leak

[Andy Lutomirski]

On Tue, Jun 14, 2016 at 2:37 PM, Dave Hansen
<dave.hansen@xxxxxxxxxxxxxxx> wrote:
> On 06/14/2016 01:16 PM, Nadav Amit wrote:
>> Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> wrote:
>>
>>> On 06/14/2016 09:47 AM, Nadav Amit wrote:
>>>> Lukasz Anaczkowski <lukasz.anaczkowski@xxxxxxxxx> wrote:
>>>>
>>>>>> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>>>>>> +void fix_pte_leak(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
>>>>>> +{
>>>> Here there should be a call to smp_mb__after_atomic() to synchronize with
>>>> switch_mm. I submitted a similar patch, which is still pending (hint).
>>>>
>>>>>> + if (cpumask_any_but(mm_cpumask(mm), smp_processor_id()) < nr_cpu_ids) {
>>>>>> +         trace_tlb_flush(TLB_LOCAL_SHOOTDOWN, TLB_FLUSH_ALL);
>>>>>> +         flush_tlb_others(mm_cpumask(mm), mm, addr,
>>>>>> +                          addr + PAGE_SIZE);
>>>>>> +         mb();
>>>>>> +         set_pte(ptep, __pte(0));
>>>>>> + }
>>>>>> +}
>>>
>>> Shouldn't that barrier be incorporated in the TLB flush code itself and
>>> not every single caller (like this code is)?
>>>
>>> It is insane to require individual TLB flushers to be concerned with the
>>> barriers.
>>
>> IMHO it is best to use existing flushing interfaces instead of creating
>> new ones.
>
> Yeah, or make these things a _little_ harder to get wrong.  That little
> snippet above isn't so crazy that we should be depending on open-coded
> barriers to get it right.
>
> Should we just add a barrier to mm_cpumask() itself?  That should stop
> the race.  Or maybe we need a new primitive like:
>
> /*
>  * Call this if a full barrier has been executed since the last
>  * pagetable modification operation.
>  */
> static int __other_cpus_need_tlb_flush(struct mm_struct *mm)
> {
>         /* cpumask_any_but() returns >= nr_cpu_ids if no cpus set. */
>         return cpumask_any_but(mm_cpumask(mm), smp_processor_id()) <
>                 nr_cpu_ids;
> }
>
>
> static int other_cpus_need_tlb_flush(struct mm_struct *mm)
> {
>         /*
>          * Synchronizes with switch_mm.  Makes sure that we do not
>          * observe a bit having been cleared in mm_cpumask() before
>          * the other processor has seen our pagetable update.  See
>          * switch_mm().
>          */
>         smp_mb__after_atomic();
>
>         return __other_cpus_need_tlb_flush(mm)
> }
>
> We should be able to deploy other_cpus_need_tlb_flush() in most of the
> cases where we are doing "cpumask_any_but(mm_cpumask(mm),
> smp_processor_id()) < nr_cpu_ids".

IMO this is a bit nuts.  smp_mb__after_atomic() doesn't do anything on
x86.  And, even if it did, why should the flush code assume that the
previous store was atomic?

What's the issue being fixed / worked around here?