Re: [PATCH 2/3] rust: maple_tree: add MapleTree::lock() and load()

[Gary Guo <gary@garyguo.net>]

On Sat, 26 Jul 2025 13:23:23 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> To load a value, one must be careful to hold the lock while accessing
> it. To enable this, we add a lock() method so that you can perform
> operations on the value before the spinlock is released.
> 
> Co-developed-by: Andrew Ballance <andrewjballance@gmail.com>
> Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/maple_tree.rs | 94 +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 94 insertions(+)
> 
> diff --git a/rust/kernel/maple_tree.rs b/rust/kernel/maple_tree.rs
> index 0f26c173eedc7c79bb8e2b56fe85e8a266b3ae0c..c7ef504a9c78065b3d5752b4f5337fb6277182d1 100644
> --- a/rust/kernel/maple_tree.rs
> +++ b/rust/kernel/maple_tree.rs
> @@ -206,6 +206,23 @@ pub fn erase(&self, index: usize) -> Option<T> {
>          unsafe { T::try_from_foreign(ret) }
>      }
>  
> +    /// Lock the internal spinlock.
> +    #[inline]
> +    pub fn lock(&self) -> MapleLock<'_, T> {
> +        // SAFETY: It's safe to lock the spinlock in a maple tree.
> +        unsafe { bindings::spin_lock(self.ma_lock()) };
> +
> +        // INVARIANT: We just took the spinlock.
> +        MapleLock(self)
> +    }
> +
> +    #[inline]
> +    fn ma_lock(&self) -> *mut bindings::spinlock_t {
> +        // SAFETY: This pointer offset operation stays in-bounds.
> +        let lock = unsafe { &raw mut (*self.tree.get()).__bindgen_anon_1.ma_lock };
> +        lock.cast()
> +    }

Could this return `&SpinLock<()>` using `Lock::from_raw`?

I guess it has the drawback of having `MapleLock` needing to store
`ma_lock` pointer but the guard is usually just all on stack and
inlined so it probably won't make a difference?

This way you remove `unsafe` from `lock` and gets a free `drop`.

> +
>      /// Free all `T` instances in this tree.
>      ///
>      /// # Safety
> @@ -248,6 +265,83 @@ fn drop(mut self: Pin<&mut Self>) {
>      }
>  }
>  
> +/// A reference to a [`MapleTree`] that owns the inner lock.
> +///
> +/// # Invariants
> +///
> +/// This guard owns the inner spinlock.
> +pub struct MapleLock<'tree, T: ForeignOwnable>(&'tree MapleTree<T>);
> +
> +impl<'tree, T: ForeignOwnable> Drop for MapleLock<'tree, T> {
> +    #[inline]
> +    fn drop(&mut self) {
> +        // SAFETY: By the type invariants, we hold this spinlock.
> +        unsafe { bindings::spin_unlock(self.0.ma_lock()) };
> +    }
> +}