Re: [PATCH 1/2] block: Add post_release() operation

["Gary Guo" <gary@garyguo.net>]

On Wed Sep 9, 2026 at 11:48 AM BST, Tetsuo Handa wrote:
> Add post_release() block device operation which provides a hook for
> performing synchronous cleanup without disk->open_mutex held, which is
> needed by the loop devices.
>
> Real-world container engines, test suites, and system utilities rely on
> fput() from __loop_clr_fd() being completed when lo_release() returns.
> But changes which went to the v7.1 merge window broke an assumption that

What change? You should mention it with a Fixes tag.

The message should also explain how the assumption is broken, not just stating
that it's broken.

> there is no outstanding I/O when __loop_clr_fd() is called, causing NULL
> pointer dereference problem in lo_rw_aio().
>
> In order to fix this regression, we want to allow __loop_clr_fd() to flush
> outstanding I/O. But calling drain_workqueue() from __loop_clr_fd() with
> disk->open_mutex held causes lockdep warnings. We need a mechanism which
> can flush outstanding I/O without disk->open_mutex held.
>
> This post_release() operation is intended for performing only idempotent
> actions such as flush_work(), for nothing prevents multiple threads from
> concurrently calling this operation. That is, the loop device schedules
> a work_struct for calling __loop_clr_fd() from lo_release() where
> disk->open_mutex is held, and waits for completion of that work_struct
> using post_release() operation where disk->open_mutex is not held.
>
> Also, this post_release() operation is called from only bdev_release()
> path. This is because loop_configure() is not yet called (there is nothing
> to clear) if something went wrong between an initialization lo_open() and
> an error-unwinding lo_release() within the bdev_open() path.
>
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
>  block/bdev.c                     | 2 ++
>  include/linux/blkdev.h           | 6 ++++++
>  rust/kernel/block/mq/gen_disk.rs | 1 +
>  3 files changed, 9 insertions(+)
>
> diff --git a/block/bdev.c b/block/bdev.c
> index cd8323083740..7ce5acaacf43 100644
> --- a/block/bdev.c
> +++ b/block/bdev.c
> @@ -1188,6 +1188,8 @@ void bdev_release(struct file *bdev_file)
>  	else
>  		blkdev_put_whole(bdev);
>  	mutex_unlock(&disk->open_mutex);
> +	if (bdev->bd_disk->fops->post_release)
> +		bdev->bd_disk->fops->post_release(bdev->bd_disk);
>  
>  	module_put(disk->fops->owner);
>  put_no_open:
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 4f7905c3412b..f05dba1b5962 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1605,6 +1605,12 @@ struct block_device_operations {
>  	 * driver.
>  	 */
>  	int (*alternative_gpt_sector)(struct gendisk *disk, sector_t *sector);
> +	/*
> +	 * Called after disk->open_mutex is released in the bdev_release() path.
> +	 * Used by loop devices that need to perform synchronization without
> +	 * holding disk->open_mutex. This operation has to be idempotent.
> +	 */
> +	void (*post_release)(struct gendisk *disk);
>  };
>  
>  #ifdef CONFIG_COMPAT
> diff --git a/rust/kernel/block/mq/gen_disk.rs b/rust/kernel/block/mq/gen_disk.rs
> index fc97dd873974..2ff77ef49781 100644
> --- a/rust/kernel/block/mq/gen_disk.rs
> +++ b/rust/kernel/block/mq/gen_disk.rs
> @@ -129,6 +129,7 @@ pub fn build<T: Operations>(
>              submit_bio: None,
>              open: None,
>              release: None,
> +            post_release: None,
>              ioctl: None,
>              compat_ioctl: None,
>              check_events: None,

You can use

    ..pin_init::zeroed()

to have all other fields zeroed. It might be beneficial if we care about knowing
all new callbacks, but it doesn't look this is the new case.

We probably should remove all `None` assignments so it's all covered by the
`..zeroed()`, but that'll need to be its own patch.

Best,
Gary