Re: [PATCH 6.12 0630/1536] rust: block: fix `Send` bound for `GenDisk`

[Miguel Ojeda <ojeda@kernel.org>]

On Sat, 07 Nov 2026 08:44:06 -0500 Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:
>
> 6.12-stable review patch.  If anyone has any objections, please let me know.
>
> ------------------
>
> From: Andreas Hindborg <a.hindborg@kernel.org>
>
> commit e502b667ee70a7fa2a578e7722672ecd4fe0eb81 upstream.
>
> The `Send` implementation for `GenDisk<T>` was conditioned on `T: Send`.
> This constrains the wrong type. `T` is the `Operations` implementation,
> which is typically a zero-sized marker type that carries no data, so `T:
> Send` says nothing about whether the data a `GenDisk` actually owns can be
> moved to another thread.
>
> A `GenDisk<T>` owns the queue data `T::QueueData` (stored as the
> `gendisk`'s `queuedata` and dropped when the `GenDisk` is dropped) and an
> `Arc<TagSet<T>>`. These are the values transferred when a `GenDisk` is sent
> across a thread boundary, so the `Send` bound must constrain exactly them.
> Bound `T::QueueData: Send` and `Arc<TagSet<T>>: Send` instead.
>
> Fixes: 3253aba3408a ("rust: block: introduce `kernel::block::mq` module")
> Cc: stable@vger.kernel.org
> Reported-by: Priya Bala Govindasamy <pgovind2@uci.edu>, Dylan Zueck <dzueck@uci.edu>, Yuan Tan <ytan089@ucr.edu>
> Closes: https://lore.kernel.org/all/cover.1780633578.git.ytan089@ucr.edu
> Signed-off-by: Yuan Tan <ytan089@ucr.edu>
> Link: https://msgid.link/20260709100100.604252-1-yuantan098@gmail.com
> [ Andreas: Fix tags to make checkpatch happy. Change summary line. ]
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
> Link: https://patch.msgid.link/20260929-rust-block-for-v7-4-rc1-b4-v1-4-642a4c1aebd7@kernel.org
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

This one breaks most Rust builds:

    error[E0220]: associated type `QueueData` not found for `T`
       --> rust/kernel/block/mq/gen_disk.rs:198:8
        |
    198 |     T::QueueData: Send,
        |        ^^^^^^^^^ associated type `QueueData` not found

because there is no `QueueData` in 6.12.y. We would also need commit
06dad60393f2 ("rust: block: implement `Send` and `Sync` for `TagSet`")
for the other bound anyway.

I would suggest dropping the patch for the moment.

Andreas: could you please take a look and see how you would like to
resolve it, if at all?

Thanks!

Cc: Andreas Hindborg <a.hindborg@kernel.org>
Cc: Boqun Feng <boqun@kernel.org>
Cc: linux-block@vger.kernel.org
Cc: rust-for-linux@vger.kernel.org

Cheers,
Miguel