linux-kernel - Re: [PATCH 1/2] rust: sync: refactor static_lock

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Message-Id: <DBJIDFSMYASO.3VRN4ZZEUI8EX@kernel.org>
Date: Wed, 23 Jul 2025 16:36:05 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>, "Boqun Feng"
 <boqun.feng@...il.com>, "Miguel Ojeda" <ojeda@...nel.org>
Cc: "Gary Guo" <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Trevor Gross" <tmgross@...ch.edu>, "Danilo Krummrich" <dakr@...nel.org>,
 <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] rust: sync: refactor static_lock_class!() macro

On Wed Jul 23, 2025 at 1:49 PM CEST, Alice Ryhl wrote:
>  impl LockClassKey {
> +    /// Initializes a statically allocated lock class key.
> +    ///
> +    /// This is usually used indirectly through the [`static_lock_class!`] macro.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The destructor must never run on the returned `LockClassKey`.

I don't know how lockdep works, but Boqun mentioned in the other thread
that it uses the address of static keys. But AFAIK there is no mechanism
to differentiate them, so does lockdep just check the address and if it
is in a static segment it uses different behavior?

Because from the safety requirements on this function, I could just do
this:

    // SAFETY: we leak the box below, so the destructor never runs.
    let class = KBox::new(unsafe { LockClassKey::new_static() });
    let class = Pin::static_ref(KBox::leak(class));
    let lock = SpinLock::new(42, c_str!("test"), class);
    let _ = lock.lock();

Because if lockdep then expects this to be initialized, we need to
change the requirement to only be used from statics.

---
Cheers,
Benno

> +    #[doc(hidden)]
> +    pub const unsafe fn new_static() -> Self {
> +        LockClassKey {
> +            inner: Opaque::uninit(),
> +        }
> +    }