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Message-ID: <2617ec4a-6413-4341-b4fe-c30c1b14432e@proton.me>
Date: Tue, 06 Aug 2024 09:23:55 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Alice Ryhl <aliceryhl@...gle.com>
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 06.08.24 10:30, Alice Ryhl wrote:
> 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.
SGTM
>
>
>>> + _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?
Do you mean on this version or in a previous one? If you mean in this
one, then I would argue that they are not "reverses" of each other. For
this instance I would prefer
// SAFETY: Due to `self._tree`, `k` and `v` are valid for the lifetime of `'a`.
self.iter_raw.next().map(|(k, v)| unsafe { (&*k, &*v) })
or
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 hope that this seems consistent, my motivation behind the suggestions
are that I don't like the comment splitting the single line.
---
Cheers,
Benno
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