[Bug 2045586] Re: livecd-rootfs uses losetup -P for theoretically reliable/synchronous partition setup but it's not reliable in noble

[dann frazier]

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);

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https://bugs.launchpad.net/bugs/2045586

Title:
  livecd-rootfs uses losetup -P for theoretically reliable/synchronous
  partition setup but it's not reliable in noble

Status in linux package in Ubuntu:
  New
Status in livecd-rootfs package in Ubuntu:
  New
Status in util-linux package in Ubuntu:
  New

Bug description:
  In mantic, we migrated livecd-rootfs to use losetup -P instead of
  kpartx, with the expectation that this would give us a reliable, race-
  free way of loop-mounting partitions from a disk image during image
  build.

  In noble, we are finding that it is no longer reliable, and in fact
  fails rather often.

  It is most noticeable with riscv64 builds, which is the architecture
  where we most frequently ran into problems before with kpartx.  The
  first riscv64+generic build in noble where the expected loop partition
  device is not available is

    https://launchpad.net/~ubuntu-
  cdimage/+livefs/ubuntu/noble/cpc/+build/531790

  The failure is however not unique to riscv64, and the autopkgtest for
  the latest version of livecd-rootfs (24.04.7) - an update that
  specifically tries to add more debugging code for this scenario - has
  also failed on ppc64el.

    https://autopkgtest.ubuntu.com/packages/l/livecd-
  rootfs/noble/ppc64el

  The first failure happened on November 16.  While there has been an
  update to the util-linux package in noble, this did not land until
  November 23.

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