Re: [PATCH] mm/rmap: fix potential batched TLB flush race

[Marco Elver]

On Wed, 24 Nov 2021 at 09:41, Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Marco Elver <elver@xxxxxxxxxx> writes:
>
> > On Wed, 24 Nov 2021 at 02:44, Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> >>
> >> Marco Elver <elver@xxxxxxxxxx> writes:
> >>
> >> > On Tue, 23 Nov 2021 at 08:44, Huang Ying <ying.huang@xxxxxxxxx> wrote:
> > [...]
> >> >> --- a/mm/rmap.c
> >> >> +++ b/mm/rmap.c
> >> >> @@ -633,7 +633,7 @@ static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
> >> >>          * before the PTE is cleared.
> >> >>          */
> >> >>         barrier();
> >> >> -       mm->tlb_flush_batched = true;
> >> >> +       atomic_inc(&mm->tlb_flush_batched);
> >> >
> >> > The use of barrier() and atomic needs some clarification.
> >>
> >> There are some comments above barrier() to describe why it is needed.
> >> For atomic, because the type of mm->tlb_flush_batched is atomic_t, do we
> >> need extra clarification?
> >
> > Apologies, maybe I wasn't clear enough: the existing comment tells me
> > the clearing of PTE should never happen after tlb_flush_batched is
> > set, but only the compiler is considered. However, I become suspicious
> > when I see barrier() paired with an atomic. barrier() is purely a
> > compiler-barrier and does not prevent the CPU from reordering things.
> > atomic_inc() does not return anything and is therefore unordered per
> > Documentation/atomic_t.txt.
> >
> >> > Is there a
> >> > requirement that the CPU also doesn't reorder anything after this
> >> > atomic_inc() (which is unordered)? I.e. should this be
> >> > atomic_inc_return_release() and remove barrier()?
> >>
> >> We don't have an atomic_xx_acquire() to pair with this.  So I guess we
> >> don't need atomic_inc_return_release()?
> >
> > You have 2 things stronger than unordered: atomic_read() which result
> > is used in a conditional branch, thus creating a control-dependency
> > ordering later dependent writes; and the atomic_cmpxchg() is fully
> > ordered.
> >
> > But before all that, I'd still want to understand what ordering
> > requirements you have. The current comments say only the compiler
> > needs taming, but does that mean we're fine with the CPU wildly
> > reordering things?
>
> Per my understanding, atomic_cmpxchg() is fully ordered, so we have
> strong ordering in flush_tlb_batched_pending().  And we use xchg() in
> ptep_get_and_clear() (at least for x86) which is called before
> set_tlb_ubc_flush_pending().  So we have strong ordering there too.
>
> So at least for x86, barrier() in set_tlb_ubc_flush_pending() appears
> unnecessary.  Is it needed by other architectures?

Hmm, this is not arch/ code -- this code needs to be portable.
atomic_t accessors provide arch-independent guarantees. But do the
other operations here provide any guarantees? If they don't, then I
think we have to assume unordered.