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

[Benno Lossin]

On 15.08.24 10:04, Alice Ryhl wrote:
> On Thu, Aug 15, 2024 at 9:49 AM Andreas Hindborg <nmi@xxxxxxxxxxxx> wrote:
>>
>> From: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>
>>
>> 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@xxxxxxxxxxxx>
>> 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@xxxxxxxxxxx>
> 
> LGTM. This makes me wonder if there's some design mistake in how we
> handle lock classes in Rust.

So `LockClassKey::new` doesn't initialize the `lock_class_key` and is
also movable. I think in this case we either just overlooked it or
thought that the C side would initialize it.

For those people that know about this, are there APIs that initialize
`lock_class_key` themselves? (ie not a function to initialize a lock
class key, but rather an API like `__blk_mq_alloc_disk`)
Because if it is usually expected that the class key is already
initialized, then I think we should change our abstraction.

Additionally, I think that it needs to be pinned, since it contains an
`struct hlist_node` (I might be wrong on this, but that looks and sounds
like an intrusive linked list).

Also the `new` function is probably prone for misuse, since it will
create a new lock class key every time it is run. But as I learned in
[1], the more common use-case is a single lock class key for several
locks. Therefore it might be a good idea to at least rename it to
`new_dynamic` or similar and add appropriate documentation pointing to
`static_lock_class!`.

[1]: https://rust-for-linux.zulipchat.com/#narrow/stream/288089-General/topic/.E2.9C.94.206.2E11.2E0-rc1.3A.20rust.2Fkernel.2Fblock.2Fmq.2Ers.3A.20doctest.20lock.20warning/near/460074755

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Cheers,
Benno