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

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Message-Id: <DBJOYRYFZJ5I.26IFPSP138T23@kernel.org>
Date: Wed, 23 Jul 2025 21:46:03 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>, "Alice Ryhl" <aliceryhl@...gle.com>
Cc: "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 6:20 PM CEST, Boqun Feng wrote:
> 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.

So the code below will go in the non-static code path. Why doesn't it
need to be initialized/registered? (but other cases need 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);
>
> 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.

Could you go into more details? What is the "technically it won't
trigger UB" part about?

---
Cheers,
Benno