On 15.08.24 21:05, Benno Lossin wrote:

On 15.08.24 10:04, Alice Ryhl wrote:

quoted

On Thu, Aug 15, 2024 at 9:49 AM Andreas Hindborg [off-list ref] wrote:

quoted

From: Andreas Hindborg <redacted>

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)" <redacted>
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 <redacted>

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