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

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Message-Id: <DBNO0N1TDAGI.2OEWH6Y60JNYZ@kernel.org>
Date: Mon, 28 Jul 2025 13:52:08 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: "Andrew Morton" <akpm@...ux-foundation.org>, "Liam R. Howlett"
 <Liam.Howlett@...cle.com>, "Lorenzo Stoakes" <lorenzo.stoakes@...cle.com>,
 "Miguel Ojeda" <ojeda@...nel.org>, "Andrew Ballance"
 <andrewjballance@...il.com>, "Boqun Feng" <boqun.feng@...il.com>, "Gary
 Guo" <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <lossin@...nel.org>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Trevor Gross" <tmgross@...ch.edu>,
 <linux-kernel@...r.kernel.org>, <maple-tree@...ts.infradead.org>,
 <rust-for-linux@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH 2/3] rust: maple_tree: add MapleTree::lock() and load()

On Sat Jul 26, 2025 at 3:23 PM CEST, Alice Ryhl wrote:
> 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()
> +    }
> +
>      /// 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()) };
> +    }
> +}

I think in the future we also want to give users access to the mas_*() function
familiy.

I assume, MaState would represent a struct ma_state, but also carry a MapleLock
guard. And then the mas_*() functions would be methods of MaState?

In case we want to allow to release and re-acquire the lock for the same
MaState, we could probably use type states.

I wonder if this (at least partially) makes sense to have from the get-go, since
it could already be used to implement things like MapleTree::free_all_entries()
based on it.