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

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Message-ID: <CAH5fLgi-3oT8+5Krzrg5JeagJMm6-8FNVr5G-UAszuhi0qZ1xA@mail.gmail.com>
Date: Sat, 26 Jul 2025 18:15:11 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Gary Guo <gary@...yguo.net>
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>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, 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 5:50 PM Gary Guo <gary@...yguo.net> wrote:
>
> On Sat, 26 Jul 2025 13:23:23 +0000
> Alice Ryhl <aliceryhl@...gle.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@...il.com>
> > Signed-off-by: Andrew Ballance <andrewjballance@...il.com>
> > Signed-off-by: Alice Ryhl <aliceryhl@...gle.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`.

I ended up going this way to avoid the extra field in MapleLock, like
you mention.

Alice