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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <imzcbuqlhj3dhzxyk7t44pt3ufxckwlonuqpsdohxuztvk3sss@z4douqoxmxqw>
Date: Thu, 7 Aug 2025 12:15:25 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Gary Guo <gary@...yguo.net>
Cc: Alice Ryhl <aliceryhl@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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()

* Gary Guo <gary@...yguo.net> [250727 08:03]:
> On Sat, 26 Jul 2025 18:18:02 +0200
> Alice Ryhl <aliceryhl@...gle.com> wrote:
> 
> > On Sat, Jul 26, 2025 at 6:15 PM Alice Ryhl <aliceryhl@...gle.com> wrote:
> > >
> > > 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.  
> > 
> > Oh, and it also avoids assuming anything about the layout of SpinLock<()>
> > 
> > Alice
> 
> Well, `Lock::from_raw` is designed to interact with a C-side lock:
> 
> > Construct a Lock from a raw pointer
> >
> > This can be useful for interacting with a lock which was initialised outside of Rust.
> 

If it matters for future build out, the tree supports an external lock
that may not be a spinlock.  This is currently used by the mm for the
vma management, and others (although willy wants it to go away
eventually).

Thanks,
Liam