linux-kernel - Re: [PATCH 2/2] rust: block: fix wrong usage of lockdep API

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Message-ID: <20240815200738.096dca4a.gary@garyguo.net>
Date: Thu, 15 Aug 2024 20:07:38 +0100
From: Gary Guo <gary@...yguo.net>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Andreas Hindborg <nmi@...aspace.dk>, Jens Axboe <axboe@...nel.dk>,
 Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Wedson Almeida Filho <wedsonaf@...il.com>, Andreas Hindborg
 <a.hindborg@...sung.com>, "Behme Dirk (XC-CP/ESB5)"
 <Dirk.Behme@...bosch.com>, Boqun Feng <boqun.feng@...il.com>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, Benno Lossin
 <benno.lossin@...ton.me>, "linux-block@...r.kernel.org"
 <linux-block@...r.kernel.org>, rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] rust: block: fix wrong usage of lockdep API

On Thu, 15 Aug 2024 10:04:56 +0200
Alice Ryhl <aliceryhl@...gle.com> wrote:

> On Thu, Aug 15, 2024 at 9:49 AM Andreas Hindborg <nmi@...aspace.dk> wrote:
> >
> > From: Andreas Hindborg <a.hindborg@...sung.com>
> >
> > When allocating `struct gendisk`, `GenDiskBuilder` is using a dynamic lock
> > class key without registering the key. This is incorrect use of the API,
> > which causes a `WARN` trace. This patch fixes the issue by using a static
> > lock class key, which is more appropriate for the situation anyway.
> >
> > Fixes: 3253aba3408a ("rust: block: introduce `kernel::block::mq` module")
> > Reported-by: "Behme Dirk (XC-CP/ESB5)" <Dirk.Behme@...bosch.com>
> > Closes: https://rust-for-linux.zulipchat.com/#narrow/stream/288089-General/topic/6.2E11.2E0-rc1.3A.20rust.2Fkernel.2Fblock.2Fmq.2Ers.3A.20doctest.20lock.20warning
> > Signed-off-by: Andreas Hindborg <a.hindborg@...sung.com>  
> 
> LGTM. This makes me wonder if there's some design mistake in how we
> handle lock classes in Rust.
> 
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>

I agree. The API that we current have is designed without much
consideration into dynamically allocated keys, and we use `&'static
LockClassKey` in a lot of kernel crate APIs.

This arguably is wrong, because presence of `&'static LockClassKey`
doesn't mean the key is static. If we do a
`Box::leak(Box::new(LockClassKey::new()))`, then this is a `&'static
LockClassKey`, but lockdep wouldn't consider this as a static object.

Maybe we should make the `new` function unsafe.

For the patch itself:

Reviewed-by: Gary Guo <gary@...yguo.net>