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

Daan De Meyer
2026-03-31T10:51:46Z

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>
---
block/disk-events.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block/disk-events.c b/block/disk-events.c
index 9f9f9f8a2d6b..074731ecc3d2 100644
--- a/block/disk-events.c
+++ b/block/disk-events.c
@@ -290,13 +290,14 @@ EXPORT_SYMBOL(disk_check_media_change);
* Should be called when the media changes for @disk. Generates a uevent
* and attempts to free all dentries and inodes and invalidates all block
* device page cache entries in that case.
+ *
+ * Callers that need a partition re-scan should arrange for one explicitly.
*/
void disk_force_media_change(struct gendisk *disk)
{
disk_event_uevent(disk, DISK_EVENT_MEDIA_CHANGE);
inc_diskseq(disk);
bdev_mark_dead(disk->part0, true);
- set_bit(GD_NEED_PART_SCAN, &disk->state);
}
EXPORT_SYMBOL_GPL(disk_force_media_change);

--
2.53.0