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

[Benno Lossin]

On 15.08.24 21:05, Benno Lossin wrote:
> 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.

Sorry, I got confused, this has nothing to do with initialization.

---
Cheers,
Benno

> 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
> 
> ---
> Cheers,
> Benno
> 
>