Re: [PATCH v3 1/3] loop: fix partition scan race between udev and loop_reread_partitions()

[Christian Brauner <brauner@kernel.org>]

On Tue, Mar 31, 2026 at 10:51:28AM +0000, Daan De Meyer wrote:
> When LOOP_CONFIGURE is called with LO_FLAGS_PARTSCAN, the following
> sequence occurs:
> 
>   1. disk_force_media_change() sets GD_NEED_PART_SCAN
>   2. Uevent suppression is lifted and a KOBJ_CHANGE uevent is sent
>   3. loop_global_unlock() releases the lock
>   4. loop_reread_partitions() calls bdev_disk_changed() to scan
> 
> There is a race between steps 2 and 4: when udev receives the uevent
> and opens the device before loop_reread_partitions() runs,
> blkdev_get_whole() in bdev.c sees GD_NEED_PART_SCAN set and calls
> bdev_disk_changed() for a first scan. Then loop_reread_partitions()
> does a second scan. The open_mutex serializes these two scans, but
> does not prevent both from running.
> 
> The second scan in bdev_disk_changed() drops all partition devices
> from the first scan (via blk_drop_partitions()) before re-adding
> them, causing partition block devices to briefly disappear. This
> breaks any systemd unit with BindsTo= on the partition device: systemd
> observes the device going dead, fails the dependent units, and does
> not retry them when the device reappears.
> 
> Fix this by removing the GD_NEED_PART_SCAN set from
> disk_force_media_change() entirely. None of the current callers need
> the lazy on-open partition scan triggered by this flag:
> 
>   - floppy: sets GENHD_FL_NO_PART, so disk_has_partscan() is always
>     false and GD_NEED_PART_SCAN has no effect.
>   - loop (loop_configure, loop_change_fd): when LO_FLAGS_PARTSCAN is
>     set, loop_reread_partitions() performs an explicit scan. When not
>     set, GD_SUPPRESS_PART_SCAN prevents the lazy scan path.
>   - loop (__loop_clr_fd): calls bdev_disk_changed() explicitly if
>     LO_FLAGS_PARTSCAN is set.
>   - nbd (nbd_clear_sock_ioctl): capacity is set to zero immediately
>     after; nbd manages GD_NEED_PART_SCAN explicitly elsewhere.
> 
> With GD_NEED_PART_SCAN no longer set by disk_force_media_change(),
> udev opening the loop device after the uevent no longer triggers a
> redundant scan in blkdev_get_whole(), and only the single explicit
> scan from loop_reread_partitions() runs.
> 
> A regression test for this bug has been submitted to blktests:
> https://github.com/linux-blktests/blktests/pull/240.
> 
> Fixes: 9f65c489b68d ("loop: raise media_change event")
> Signed-off-by: Daan De Meyer <daan@amutable.com>
> ---

Looks good to me,
Acked-by: Christian Brauner <brauner@kernel.org>