Re: [PATCH v3 tip/perf/core 1/4] mm: introduce mmap_lock_speculation_{start|end}

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Oct 23, 2024 at 1:10 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Oct 10, 2024 at 01:56:41PM -0700, Andrii Nakryiko wrote:
> > From: Suren Baghdasaryan <surenb@google.com>
> >
> > Add helper functions to speculatively perform operations without
> > read-locking mmap_lock, expecting that mmap_lock will not be
> > write-locked and mm is not modified from under us.
> >
> > Suggested-by: Peter Zijlstra <peterz@infradead.org>
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > Link: https://lore.kernel.org/bpf/20240912210222.186542-1-surenb@google.com
> > ---
> >  include/linux/mm_types.h  |  3 ++
> >  include/linux/mmap_lock.h | 72 ++++++++++++++++++++++++++++++++-------
> >  kernel/fork.c             |  3 --
> >  3 files changed, 63 insertions(+), 15 deletions(-)
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 6e3bdf8e38bc..5d8cdebd42bc 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -887,6 +887,9 @@ struct mm_struct {
> >                * Roughly speaking, incrementing the sequence number is
> >                * equivalent to releasing locks on VMAs; reading the sequence
> >                * number can be part of taking a read lock on a VMA.
> > +              * Incremented every time mmap_lock is write-locked/unlocked.
> > +              * Initialized to 0, therefore odd values indicate mmap_lock
> > +              * is write-locked and even values that it's released.
> >                *
> >                * Can be modified under write mmap_lock using RELEASE
> >                * semantics.
> > diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> > index de9dc20b01ba..9d23635bc701 100644
> > --- a/include/linux/mmap_lock.h
> > +++ b/include/linux/mmap_lock.h
> > @@ -71,39 +71,84 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
> >  }
> >
> >  #ifdef CONFIG_PER_VMA_LOCK
> > +static inline void init_mm_lock_seq(struct mm_struct *mm)
> > +{
> > +     mm->mm_lock_seq = 0;
> > +}
> > +
> >  /*
> > - * Drop all currently-held per-VMA locks.
> > - * This is called from the mmap_lock implementation directly before releasing
> > - * a write-locked mmap_lock (or downgrading it to read-locked).
> > - * This should normally NOT be called manually from other places.
> > - * If you want to call this manually anyway, keep in mind that this will release
> > - * *all* VMA write locks, including ones from further up the stack.
> > + * Increment mm->mm_lock_seq when mmap_lock is write-locked (ACQUIRE semantics)
> > + * or write-unlocked (RELEASE semantics).
> >   */
> > -static inline void vma_end_write_all(struct mm_struct *mm)
> > +static inline void inc_mm_lock_seq(struct mm_struct *mm, bool acquire)
> >  {
> >       mmap_assert_write_locked(mm);
> >       /*
> >        * Nobody can concurrently modify mm->mm_lock_seq due to exclusive
> >        * mmap_lock being held.
> > -      * We need RELEASE semantics here to ensure that preceding stores into
> > -      * the VMA take effect before we unlock it with this store.
> > -      * Pairs with ACQUIRE semantics in vma_start_read().
> >        */
> > -     smp_store_release(&mm->mm_lock_seq, mm->mm_lock_seq + 1);
> > +
> > +     if (acquire) {
> > +             WRITE_ONCE(mm->mm_lock_seq, mm->mm_lock_seq + 1);
> > +             /*
> > +              * For ACQUIRE semantics we should ensure no following stores are
> > +              * reordered to appear before the mm->mm_lock_seq modification.
> > +              */
> > +             smp_wmb();
>
> Strictly speaking this isn't ACQUIRE, nor do we care about ACQUIRE here.
> This really is about subsequent stores, loads are irrelevant.
>
> > +     } else {
> > +             /*
> > +              * We need RELEASE semantics here to ensure that preceding stores
> > +              * into the VMA take effect before we unlock it with this store.
> > +              * Pairs with ACQUIRE semantics in vma_start_read().
> > +              */
>
> Again, not strictly true. We don't care about loads. Using RELEASE here
> is fine and probably cheaper on a few platforms, but we don't strictly
> need/care about RELEASE.
>
> > +             smp_store_release(&mm->mm_lock_seq, mm->mm_lock_seq + 1);
> > +     }
> > +}
>
> Also, it might be saner to stick closer to the seqcount naming of
> things and use two different functions for these two different things.
>
> /* straight up copy of do_raw_write_seqcount_begin() */
> static inline void mm_write_seqlock_begin(struct mm_struct *mm)
> {
>         kcsan_nestable_atomic_begin();
>         mm->mm_lock_seq++;
>         smp_wmb();
> }
>
> /* straigjt up copy of do_raw_write_seqcount_end() */
> static inline void mm_write_seqcount_end(struct mm_struct *mm)
> {
>         smp_wmb();
>         mm->mm_lock_seq++;
>         kcsan_nestable_atomic_end();
> }
>
> Or better yet, just use seqcount...

Yeah, with these changes it does look a lot like seqcount now...
I can take another stab at rewriting this using seqcount_t but one
issue that Jann was concerned about is the counter being int vs long.
seqcount_t uses unsigned, so I'm not sure how to address that if I
were to use seqcount_t. Any suggestions how to address that before I
move forward with a rewrite?

>
> > +
> > +static inline bool mmap_lock_speculation_start(struct mm_struct *mm, int *seq)
> > +{
> > +     /* Pairs with RELEASE semantics in inc_mm_lock_seq(). */
> > +     *seq = smp_load_acquire(&mm->mm_lock_seq);
> > +     /* Allow speculation if mmap_lock is not write-locked */
> > +     return (*seq & 1) == 0;
> > +}
> > +
> > +static inline bool mmap_lock_speculation_end(struct mm_struct *mm, int seq)
> > +{
> > +     /* Pairs with ACQUIRE semantics in inc_mm_lock_seq(). */
> > +     smp_rmb();
> > +     return seq == READ_ONCE(mm->mm_lock_seq);
> >  }
>
> Because there's nothing better than well known functions with a randomly
> different name and interface I suppose...
>
>
> Anyway, all the actual code proposed is not wrong. I'm just a bit
> annoyed its a random NIH of seqcount.

Ack. Let's decide what we do about u32 vs u64 issue and I'll rewrite this.