Re: [PATCH 2/8] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from Andy Lutomirski's message of December 3, 2020 3:09 pm:
> On Tue, Dec 1, 2020 at 6:50 PM Nicholas Piggin <npiggin@gmail.com> wrote:
>>
>> Excerpts from Andy Lutomirski's message of November 29, 2020 3:55 am:
>> > On Sat, Nov 28, 2020 at 8:02 AM Nicholas Piggin <npiggin@gmail.com> wrote:
>> >>
>> >> And get rid of the generic sync_core_before_usermode facility. This is
>> >> functionally a no-op in the core scheduler code, but it also catches
>> >>
>> >> This helper is the wrong way around I think. The idea that membarrier
>> >> state requires a core sync before returning to user is the easy one
>> >> that does not need hiding behind membarrier calls. The gap in core
>> >> synchronization due to x86's sysret/sysexit and lazy tlb mode, is the
>> >> tricky detail that is better put in x86 lazy tlb code.
>> >>
>> >> Consider if an arch did not synchronize core in switch_mm either, then
>> >> membarrier_mm_sync_core_before_usermode would be in the wrong place
>> >> but arch specific mmu context functions would still be the right place.
>> >> There is also a exit_lazy_tlb case that is not covered by this call, which
>> >> could be a bugs (kthread use mm the membarrier process's mm then context
>> >> switch back to the process without switching mm or lazy mm switch).
>> >>
>> >> This makes lazy tlb code a bit more modular.
>> >
>> > I have a couple of membarrier fixes that I want to send out today or
>> > tomorrow, and they might eliminate the need for this patch.  Let me
>> > think about this a little bit.  I'll cc you.  The existing code is way
>> > to subtle and the comments are far too confusing for me to be quickly
>> > confident about any of my conclusions :)
>> >
>>
>> Thanks for the head's up. I'll have to have a better look through them
>> but I don't know that it eliminates the need for this entirely although
>> it might close some gaps and make this not a bug fix. The problem here
>> is x86 code wanted something to be called when a lazy mm is unlazied,
>> but it missed some spots and also the core scheduler doesn't need to
>> know about those x86 details if it has this generic call that annotates
>> the lazy handling better.
> 
> I'll send v3 tomorrow.  They add more sync_core_before_usermode() callers.
> 
> Having looked at your patches a bunch and the membarrier code a bunch,
> I don't think I like the approach of pushing this logic out into new
> core functions called by arch code.  Right now, even with my
> membarrier patches applied, understanding how (for example) the x86
> switch_mm_irqs_off() plus the scheduler code provides the barriers
> that membarrier needs is quite complicated, and it's not clear to me
> that the code is even correct.  At the very least I'm pretty sure that
> the x86 comments are misleading.
>
> With your patches, someone trying to
> audit the code would have to follow core code calling into arch code
> and back out into core code to figure out what's going on.  I think
> the result is worse.

Sorry I missed this and rather than reply to the later version you
have a bigger comment here.

I disagree. Until now nobody following it noticed that the mm gets
un-lazied in other cases, because that was not too clear from the
code (only indirectly using non-standard terminology in the arch
support document).

In other words, membarrier needs a special sync to deal with the case 
when a kthread takes the mm. exit_lazy_tlb gives membarrier code that 
exact hook that it wants from the core scheduler code.

> 
> I wrote this incomplete patch which takes the opposite approach (sorry
> for whitespace damage):

That said, if you want to move the code entirely in the x86 arch from
exit_lazy_tlb to switch_mm_irqs_off, it's trivial and touches no core
code after my series :) and I would have no problem with doing that.

I suspect it might actually be more readable to go the other way and
pull most of the real_prev == next membarrier code into exit_lazy_tlb
instead, but I could be wrong I don't know exactly how the x86 lazy
state correlates with core lazy tlb state.

Thanks,
Nick

> 
> commit 928b5c60e93f475934892d6e0b357ebf0a2bf87d
> Author: Andy Lutomirski <luto@kernel.org>
> Date:   Wed Dec 2 17:24:02 2020 -0800
> 
>     [WIP] x86/mm: Handle unlazying membarrier core sync in the arch code
> 
>     The core scheduler isn't a great place for
>     membarrier_mm_sync_core_before_usermode() -- the core scheduler
>     doesn't actually know whether we are lazy.  With the old code, if a
>     CPU is running a membarrier-registered task, goes idle, gets unlazied
>     via a TLB shootdown IPI, and switches back to the
>     membarrier-registered task, it will do an unnecessary core sync.
> 
>     Conveniently, x86 is the only architecture that does anything in this
>     hook, so we can just move the code.
> 
>     XXX: actually delete the old code.
> 
>     Signed-off-by: Andy Lutomirski <luto@kernel.org>
> 
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 3338a1feccf9..e27300fc865b 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -496,6 +496,8 @@ void switch_mm_irqs_off(struct mm_struct *prev,
> struct mm_struct *next,
>           * from one thread in a process to another thread in the same
>           * process. No TLB flush required.
>           */
> +
> +        // XXX: why is this okay wrt membarrier?
>          if (!was_lazy)
>              return;
> 
> @@ -508,12 +510,24 @@ void switch_mm_irqs_off(struct mm_struct *prev,
> struct mm_struct *next,
>          smp_mb();
>          next_tlb_gen = atomic64_read(&next->context.tlb_gen);
>          if (this_cpu_read(cpu_tlbstate.ctxs[prev_asid].tlb_gen) ==
> -                next_tlb_gen)
> +            next_tlb_gen) {
> +            /*
> +             * We're reactivating an mm, and membarrier might
> +             * need to serialize.  Tell membarrier.
> +             */
> +
> +            // XXX: I can't understand the logic in
> +            // membarrier_mm_sync_core_before_usermode().  What's
> +            // the mm check for?
> +            membarrier_mm_sync_core_before_usermode(next);
>              return;
> +        }
> 
>          /*
>           * TLB contents went out of date while we were in lazy
>           * mode. Fall through to the TLB switching code below.
> +         * No need for an explicit membarrier invocation -- the CR3
> +         * write will serialize.
>           */
>          new_asid = prev_asid;
>          need_flush = true;
>