linux-kernel - Re: [PATCH 10/10] rust: xarray: fix false positive lockdep warnings

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Message-ID: <87a4y6u3dm.fsf@t14s.mail-host-address-is-not-set>
Date: Wed, 21 Jan 2026 21:17:41 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: Gary Guo <gary@...yguo.net>, Gary Guo <gary@...yguo.net>, Tamir
 Duberstein <tamird@...il.com>, Miguel Ojeda <ojeda@...nel.org>, Alex
 Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>,
 Björn
 Roy Baron <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>,
 Alice
 Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, Danilo
 Krummrich <dakr@...nel.org>
Cc: Daniel Gomez <da.gomez@...nel.org>, rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/10] rust: xarray: fix false positive lockdep warnings

"Gary Guo" <gary@...yguo.net> writes:

> On Wed Jan 21, 2026 at 7:01 PM GMT, Andreas Hindborg wrote:
>> "Gary Guo" <gary@...yguo.net> writes:
>>
>>> On Wed Dec 3, 2025 at 10:26 PM GMT, Andreas Hindborg wrote:
>>>> Replace the `xa_init_flags` helper with direct initialization of XArray
>>>> structures using `__spin_lock_init`. This allows each XArray to have
>>>> its own lock class key for lockdep, preventing false positive warnings
>>>> about lock ordering violations.
>>>>
>>>
>>> Isn't this potentially a problem on the C side as well? `xa_init_flags` is a
>>> static inline function, which means that the lock class is going to be the same
>>> if a single C compilation unit initializes multiple xarrays -- unlike when you
>>> use spin_lock_init, where each callsite gets a different lock class.
>>>
>>> Best,
>>> Gary
>>>
>>>> Add a `new_xarray!` macro that automatically generates a unique lock
>>>> class key for each XArray instantiation site. The macro accepts an
>>>> optional name parameter and uses the `optional_name!` and
>>>> `static_lock_class!` macros to generate appropriate names and lock
>>>> classes.
>>
>> My intuition about this was that when the C static function is inlined, a
>> new static address is used for each place the function is inlined. Is
>> this not correct?
>
> No, the static inside static functions still have the normal static semantics.
> There's a single copy regardless how many times the function is inlined.
>
> So you have a unique copy per compilation unit.

I see. A any rate I get into trouble with lockdep when I lock multiple
different xarrays concurrently in the same code gen unit. I am not sure
how they solve that in C, or if it will also be a problem there.


Best regards,
Andreas Hindborg