Message-ID: <CAH5fLghJmA_01QJvzSBX4HEfLwPr+FskM9dzOs0ykbYj_sCWTg@mail.gmail.com>
Date: Tue, 6 Aug 2024 10:30:40 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Matt Gilbride <mattgilbride@...gle.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Andreas Hindborg <a.hindborg@...sung.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Arve Hjønnevåg <arve@...roid.com>, 
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, 
	Joel Fernandes <joel@...lfernandes.org>, Carlos Llamas <cmllamas@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Christian Brauner <brauner@...nel.org>, Rob Landley <rob@...dley.net>, 
	Davidlohr Bueso <dave@...olabs.net>, Michel Lespinasse <michel@...pinasse.org>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 4/6] rust: rbtree: add mutable iterator

On Mon, Aug 5, 2024 at 9:22 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>
> On 27.07.24 22:30, Matt Gilbride wrote:
> > From: Wedson Almeida Filho <wedsonaf@...il.com>
> >
> > Add mutable Iterator implementation for `RBTree`,
> > allowing iteration over (key, value) pairs in key order. Only values are
> > mutable, as mutating keys implies modifying a node's position in the tree.
> >
> > Mutable iteration is used by the binder driver during shutdown to
> > clean up the tree maintained by the "range allocator" [1].
> >
> > Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-6-08ba9197f637@google.com/ [1]
> > Signed-off-by: Wedson Almeida Filho <wedsonaf@...il.com>
> > Signed-off-by: Matt Gilbride <mattgilbride@...gle.com>
> > Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> > Tested-by: Alice Ryhl <aliceryhl@...gle.com>
> > ---
> >  rust/kernel/rbtree.rs | 98 ++++++++++++++++++++++++++++++++++++++++++++-------
> >  1 file changed, 86 insertions(+), 12 deletions(-)
> >
> > diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
> > index d10074e4ac58..d7514ebadfa8 100644
> > --- a/rust/kernel/rbtree.rs
> > +++ b/rust/kernel/rbtree.rs
> > @@ -197,8 +197,26 @@ pub fn iter(&self) -> Iter<'_, K, V> {
> >          // INVARIANT: `bindings::rb_first` returns a valid pointer to a tree node given a valid pointer to a tree root.
>
> This INVARIANT is out of place, `Iter` doesn't have any INVARIANT any
> more.

We can delete it.

> >          Iter {
> >              _tree: PhantomData,
> > -            // SAFETY: `self.root` is a valid pointer to the tree root.
> > -            next: unsafe { bindings::rb_first(&self.root) },
> > +            iter_raw: IterRaw {
>
> This `IterRaw` construction is missing an INVARIANT comment. I think you
> can copy paste from below.

We can copy from below.

> > +                // SAFETY: by the invariants, all pointers are valid.
> > +                next: unsafe { bindings::rb_first(&self.root) },
> > +                _phantom: PhantomData,
> > +            },
> > +        }
> > +    }
> > +
> > +    /// Returns a mutable iterator over the tree nodes, sorted by key.
> > +    pub fn iter_mut(&mut self) -> IterMut<'_, K, V> {
> > +        IterMut {
> > +            _tree: PhantomData,
> > +            // INVARIANT:
> > +            //   - `self.root` is a valid pointer to a tree root.
> > +            //   - `bindings::rb_first` produces a valid pointer to a node given `root` is valid.
> > +            iter_raw: IterRaw {
> > +                // SAFETY: by the invariants, all pointers are valid.
> > +                next: unsafe { bindings::rb_first(&self.root) },
>
> Does this really derive a mutable reference? Ie shouldn't this be:?
>
>     next: unsafe { bindings::rb_first(&mut self.root) },

Let's change this to:

next: unsafe { bindings::rb_first(ptr::from_mut(&mut self.root)) }

This way, the pointer will be derived from a mutable reference even if
it becomes a `*const` through intermediate operations.


> > +                _phantom: PhantomData,
> > +            },
> >          }
> >      }
> >
> > @@ -211,6 +229,11 @@ pub fn keys(&self) -> impl Iterator<Item = &'_ K> {
> >      pub fn values(&self) -> impl Iterator<Item = &'_ V> {
> >          self.iter().map(|(_, v)| v)
> >      }
> > +
> > +    /// Returns a mutable iterator over the values of the nodes in the tree, sorted by key.
> > +    pub fn values_mut(&mut self) -> impl Iterator<Item = &'_ mut V> {
> > +        self.iter_mut().map(|(_, v)| v)
> > +    }
> >  }
> >
> >  impl<K, V> RBTree<K, V>
> > @@ -414,13 +437,9 @@ fn into_iter(self) -> Self::IntoIter {
> >  /// An iterator over the nodes of a [`RBTree`].
> >  ///
> >  /// Instances are created by calling [`RBTree::iter`].
> > -///
> > -/// # Invariants
> > -/// - `self.next` is a valid pointer.
> > -/// - `self.next` points to a node stored inside of a valid `RBTree`.
> >  pub struct Iter<'a, K, V> {
> >      _tree: PhantomData<&'a RBTree<K, V>>,
> > -    next: *mut bindings::rb_node,
> > +    iter_raw: IterRaw<K, V>,
> >  }
> >
> >  // SAFETY: The [`Iter`] gives out immutable references to K and V, so it has the same
> > @@ -434,21 +453,76 @@ unsafe impl<'a, K: Sync, V: Sync> Sync for Iter<'a, K, V> {}
> >  impl<'a, K, V> Iterator for Iter<'a, K, V> {
> >      type Item = (&'a K, &'a V);
> >
> > +    fn next(&mut self) -> Option<Self::Item> {
> > +        self.iter_raw.next().map(|(k, v)|
> > +            // SAFETY: Due to `self._tree`, `k` and `v` are valid for the lifetime of `'a`.
> > +            unsafe { (&*k, &*v) })
>
> I don't really like the formatting here, can you move the SAFETY one
> line upwards? It should format nicely then.

You suggested exactly the reverse formatting change on RBTreeCursor?

> > +    }
> > +}
> > +
> > +impl<'a, K, V> IntoIterator for &'a mut RBTree<K, V> {
> > +    type Item = (&'a K, &'a mut V);
> > +    type IntoIter = IterMut<'a, K, V>;
> > +
> > +    fn into_iter(self) -> Self::IntoIter {
> > +        self.iter_mut()
> > +    }
> > +}
> > +
> > +/// A mutable iterator over the nodes of a [`RBTree`].
> > +///
> > +/// Instances are created by calling [`RBTree::iter_mut`].
> > +pub struct IterMut<'a, K, V> {
> > +    _tree: PhantomData<&'a mut RBTree<K, V>>,
> > +    iter_raw: IterRaw<K, V>,
> > +}
> > +
> > +// SAFETY: The [`IterMut`] gives out immutable references to K and mutable references to V, so it has the same
> > +// thread safety requirements as mutable references.
> > +unsafe impl<'a, K: Send, V: Send> Send for IterMut<'a, K, V> {}
>
> Since we only borrow `K` immutably, would it make sense to have `K:
> Sync`?

No, `K: Send` is better because it's less restrictive in practice.

Alice

Powered by blists - more mailing lists