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

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Message-ID: <aIELxq_iVMfjszkh@tardis-2.local>
Date: Wed, 23 Jul 2025 09:20:22 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Benno Lossin <lossin@...nel.org>, 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 05:01:39PM +0200, Alice Ryhl wrote:
> 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

In lockdep, we use `static_obj()` to tell whether it's a static obj or a
dynamic allocated one.

> > 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);

This will trigger a runtime error because `class` is not static, but
technically, it won't trigger UB, at least lockdep should be able to
handle this case.

Regards,
Boqun

> >     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