f225a3b922
The kernel loop driver has a race condition in LOOP_CONFIGURE when
LO_FLAGS_PARTSCAN is set: it sends a KOBJ_CHANGE uevent (with
GD_NEED_PART_SCAN set) before calling loop_reread_partitions(). If
udev opens the device in response to the uevent before
loop_reread_partitions() runs, the kernel's blkdev_get_whole() sees
GD_NEED_PART_SCAN and triggers a first partition scan. Then
loop_reread_partitions() runs a second scan that drops all partitions
from the first scan (via blk_drop_partitions()) before re-adding them.
This causes partition devices to briefly disappear (plugged -> dead ->
plugged), which breaks systemd units with BindsTo= on the partition
device: systemd observes the dead transition, fails the dependent
units with 'dependency', and does not retry when the device reappears.
Work around this in loop_device_make_internal() by splitting the loop
device setup into two steps: first LOOP_CONFIGURE without
LO_FLAGS_PARTSCAN, then LOOP_SET_STATUS64 to enable partscan. This
avoids the race because:
1. LOOP_CONFIGURE without partscan: disk_force_media_change() sets
GD_NEED_PART_SCAN, but GD_SUPPRESS_PART_SCAN remains set. If udev
opens the device, blkdev_get_whole() calls bdev_disk_changed()
which clears GD_NEED_PART_SCAN, but blk_add_partitions() returns
early because disk_has_partscan() is false — no partitions appear,
the flag is drained harmlessly.
2. Between the two ioctls, we open and close the device to ensure
GD_NEED_PART_SCAN is drained regardless of whether udev processed
the uevent yet.
3. LOOP_SET_STATUS64 with LO_FLAGS_PARTSCAN: clears
GD_SUPPRESS_PART_SCAN and calls loop_reread_partitions() for a
single clean scan. Crucially, loop_set_status() does not call
disk_force_media_change(), so GD_NEED_PART_SCAN is never set again.
A proper kernel fix has been submitted:
https://lore.kernel.org/linux-block/20260330081819.652890-1-daan@amutable.com/T/#u
This workaround should be dropped once the fix is widely available.
Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>
(cherry picked from commit d3cb7a4e0f)
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