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

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Message-ID: <CAH5fLgjWFa8TjTL+rfv7Zd+OQqhkKqWvyTkGf60pMUyQ=c4sXg@mail.gmail.com>
Date: Wed, 23 Jul 2025 17:01:39 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <lossin@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>, Miguel Ojeda <ojeda@...nel.org>, 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 4:36 PM Benno Lossin <lossin@...nel.org> wrote:
>
> 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.

My understanding is that it has to with correctly handling reuse of
the same key. In your scenario it's not reused.

Alice