loop: Fix teardown

Hi Jens,

This series fixes loop driver teardown. When tearing down an autoclear loop device, the loop driver must drain in-flight I/O and flush workqueues to prevent NULL pointer dereferences in lo_rw_aio() and related I/O paths.

The .release() block device callback is invoked while holding disk->open_mutex. Freezing the request queue or draining workqueues under disk->open_mutex causes lock inversion and circular locking dependencies (e.g., when worker threads or I/O completion paths also acquire open_mutex or interact with request queue synchronization).

This patch series resolves the lock inversion by:
1. Adding a .post_release() block device operation that is called synchronously from bdev_release() immediately after disk->open_mutex is released.
2. Migrating __loop_clr_fd() to .post_release(), allowing loop device teardown and queue freezing to happen outside of disk->open_mutex.

This series is inspired by Tetsuo's loop driver patch series.

Please consider applying this patch series.

Thanks,

Bart.

Bart Van Assche (1):
loop: Perform __loop_clr_fd() after disk->open_mutex is dropped

Tetsuo Handa (1):
block: Add post_release() operation

 block/bdev.c                     |  3 ++
 drivers/block/loop.c             | 60 +++++++++++++++++++++++---------
 include/linux/blkdev.h           |  8 +++++
 rust/kernel/block/mq/gen_disk.rs |  1 +
 4 files changed, 56 insertions(+), 16 deletions(-)