Comment 11 for bug 2045586

I ran the above test:
  https://autopkgtest.ubuntu.com/results/autopkgtest-jammy-dannf-test/jammy/amd64/l/livecd-rootfs/20240123_035147_6470b@/log.gz

It does appear that systemd-udevd is trying to scan partitions at the same time as losetup:

1599s ++ losetup --show -f -P -v binary/boot/disk-uefi.ext4
1600s + loop_device=/dev/loop0
1600s + '[' '!' -b /dev/loop0 ']'
1600s + rootfs_dev_mapper=/dev/loop0p1
1600s + '[' '!' -b /dev/loop0p1 ']'
1600s + echo '/dev/loop0p1 is not a block device'
1600s /dev/loop0p1 is not a block device
1600s + echo '=== dmesg ==='
1600s === dmesg ===
1600s + dmesg -c
1600s [ 986.014824] EXT4-fs (loop0p1): mounted filesystem with ordered data mode. Opts: (null). Quota mode: none.
1600s [ 992.684380] EXT4-fs (loop0p1): mounted filesystem with ordered data mode. Opts: (null). Quota mode: none.
1600s [ 1043.171603] loop0: detected capacity change from 0 to 4612096
1600s [ 1043.171924] loop0: p1 p14 p15
1600s [ 1043.190421] loop0: p1 p14 p15
1600s + cat /sys/kernel/debug/tracing/trace
1600s # tracer: function
1600s #
1600s # entries-in-buffer/entries-written: 2/2 #P:4
1600s #
1600s # _-----=> irqs-off
1600s # / _----=> need-resched
1600s # | / _---=> hardirq/softirq
1600s # || / _--=> preempt-depth
1600s # ||| / _-=> migrate-disable
1600s # |||| / delay
1600s # TASK-PID CPU# ||||| TIMESTAMP FUNCTION
1600s # | | | ||||| | |
1600s losetup-50167 [002] ..... 1043.176845: bdev_disk_changed <-loop_reread_partitions
1600s systemd-udevd-321 [000] ..... 1043.195003: bdev_disk_changed <-blkdev_get_whole
1600s + echo 0
1600s + ls -l /dev/loop0p1
1600s brw------- 1 root root 259, 3 Jan 23 03:51 /dev/loop0p1
1600s + exit 1
1600s + clean_loops

Maybe we just need something like this?

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 48c530b83000e..52fda87f5d674 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1366,13 +1366,13 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
        if (partscan)
                lo->lo_disk->flags &= ~GENHD_FL_NO_PART_SCAN;

- /* enable and uncork uevent now that we are done */
- dev_set_uevent_suppress(disk_to_dev(lo->lo_disk), 0);
-
        loop_global_unlock(lo, is_loop);
        if (partscan)
                loop_reread_partitions(lo);

+ /* enable and uncork uevent now that we are done */
+ dev_set_uevent_suppress(disk_to_dev(lo->lo_disk), 0);
+
        if (!(mode & FMODE_EXCL))
                bd_abort_claiming(bdev, loop_configure);