Re: [PATCH v3 05/11] x86/mm: Track the TLB's tlb_gen and update the flushing algorithm

[Andy Lutomirski]

On Thu, Jun 22, 2017 at 7:59 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> On Thu, Jun 22, 2017 at 07:48:21AM -0700, Andy Lutomirski wrote:
>> On Thu, Jun 22, 2017 at 12:24 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
>> > On Wed, Jun 21, 2017 at 07:46:05PM -0700, Andy Lutomirski wrote:
>> >> > I'm certainly still missing something here:
>> >> >
>> >> > We have f->new_tlb_gen and mm_tlb_gen to control the flushing, i.e., we
>> >> > do once
>> >> >
>> >> >         bump_mm_tlb_gen(mm);
>> >> >
>> >> > and once
>> >> >
>> >> >         info.new_tlb_gen = bump_mm_tlb_gen(mm);
>> >> >
>> >> > and in both cases, the bumping is done on mm->context.tlb_gen.
>> >> >
>> >> > So why isn't that enough to do the flushing and we have to consult
>> >> > info.new_tlb_gen too?
>> >>
>> >> The issue is a possible race.  Suppose we start at tlb_gen == 1 and
>> >> then two concurrent flushes happen.  The first flush is a full flush
>> >> and sets tlb_gen to 2.  The second is a partial flush and sets tlb_gen
>> >> to 3.  If the second flush gets propagated to a given CPU first and it
>> >
>> > Maybe I'm still missing something, which is likely...
>> >
>> > but if the second flush gets propagated to the CPU first, the CPU will
>> > have local tlb_gen 1 and thus enforce a full flush anyway because we
>> > will go 1 -> 3 on that particular CPU. Or?
>> >
>>
>> Yes, exactly.  Which means I'm probably just misunderstanding your
>> original question.  Can you re-ask it?
>
> Ah, simple: we control the flushing with info.new_tlb_gen and
> mm->context.tlb_gen. I.e., this check:
>
>
>         if (f->end != TLB_FLUSH_ALL &&
>             f->new_tlb_gen == local_tlb_gen + 1 &&
>             f->new_tlb_gen == mm_tlb_gen) {
>
> why can't we write:
>
>         if (f->end != TLB_FLUSH_ALL &&
>             mm_tlb_gen == local_tlb_gen + 1)
>
> ?

Ah, I thought you were asking about why I needed mm_tlb_gen ==
local_tlb_gen + 1.  This is just an optimization, or at least I hope
it is.  The idea is that, if we know that another flush is coming, it
seems likely that it would be faster to do a full flush and increase
local_tlb_gen all the way to mm_tlb_gen rather than doing a partial
flush, increasing local_tlb_gen to something less than mm_tlb_gen, and
needing to flush again very soon.

>
> If mm_tlb_gen is + 2, then we'll do a full flush, if it is + 1, then
> partial.
>
> If the second flush, as you say is a partial one and still gets
> propagated first, the check will force a full flush anyway.
>
> When the first flush propagates after the second, we'll ignore it
> because local_tlb_gen has advanced adready due to the second flush.
>
> As a matter of fact, we could simplify the logic: if local_tlb_gen is
> only mm_tlb_gen - 1, then do the requested flush type.

Hmm.  I'd be nervous that there are more subtle races if we do this.
For example, suppose that a partial flush increments tlb_gen from 1 to
2 and a full flush increments tlb_gen from 2 to 3.  Meanwhile, the CPU
is busy switching back and forth between mms, so the partial flush
sees the cpu set in mm_cpumask but the full flush doesn't see the cpu
set in mm_cpumask.  The flush IPI hits after a switch_mm_irqs_off()
call notices the change from 1 to 2.  switch_mm_irqs_off() will do a
full flush and increment the local tlb_gen to 2, and the IPI handler
for the partial flush will see local_tlb_gen == mm_tlb_gen - 1
(because local_tlb_gen == 2 and mm_tlb_gen == 3) and do a partial
flush.  The problem here is that it's not obvious to me that this
actually ends up flushing everything that's needed.  Maybe all the
memory ordering gets this right, but I can imagine scenarios in which
switch_mm_irqs_off() does its flush early enough that the TLB picks up
an entry that was supposed to get zapped by the full flush.

IOW it *might* be valid, but I think it would need very careful review
and documentation.

--Andy