[Bug 2087877] [NEW] Calling "flock parted mkpart" soft deadlocks

[Chris Gunn]

Public bug reported:

In Ubuntu 24.04, if you call 'parted' or 'partprobe' while holding a
file-lock to the disk device (e.g. using 'flock'), then the 'parted'
command can end up deadlocked on the internal call to 'udevadm settle'.

It looks like back in 2013, Ubuntu added a patch to libparted that calls
`udevadm settle' both before and after partition changes are committed
to disk:

https://git.launchpad.net/ubuntu/+source/parted/tree/debian/patches/udevadm-
settle.patch

In Ubuntu 22.04, if you call 'parted' under 'flock', then it works fine.
In systemd versions prior to v251, if a udev event was triggered that
couldn't be processed because the disk's file-lock was held by another
process, then the udev event was simply dropped. This would allow the
udev event queue to clear. Which in turn would allow the 'udevadm
settle' call to complete.

However, in systemd v251 a change was made to the udev event queue where
instead of dropping udev events that are blocked by a disk's file-lock,
the udev events are instead requeued in a loop. This prevents the udev
event queue from clearing. Which in turn blocks the 'udevadm settle'
call in 'parted'.

https://github.com/systemd/systemd/commit/5d354e525a56955ae7f68062e283dda85ab07794

Systemd's official recommendation is that users should use 'flock' when
making changes to partitions. If everyone did that, then I suspect the
'udevadm-settle.patch' patch would not be needed.

  https://systemd.io/BLOCK_DEVICE_LOCKING/

However, I realize that removing the 'udevadm-settle.patch' patch might
be undesirable since it might break back-compat for users that don't use
'flock' with 'parted'.

As a workaround, the 'udevadm-settle.patch' patch could be modified to
first check if the disk file-lock is held and if it is, assume the user
is using 'flock' and don't call 'udevadm settle'. This should hopefully
fix the issue for users who use 'flock', while maintaining existing
behavior for users who don't use 'flock'.

** Affects: parted (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/2087877

Title:
  Calling "flock parted mkpart" soft deadlocks

Status in parted package in Ubuntu:
  New

Bug description:
  In Ubuntu 24.04, if you call 'parted' or 'partprobe' while holding a
  file-lock to the disk device (e.g. using 'flock'), then the 'parted'
  command can end up deadlocked on the internal call to 'udevadm
  settle'.

  It looks like back in 2013, Ubuntu added a patch to libparted that
  calls `udevadm settle' both before and after partition changes are
  committed to disk:

  https://git.launchpad.net/ubuntu/+source/parted/tree/debian/patches/udevadm-
  settle.patch

  In Ubuntu 22.04, if you call 'parted' under 'flock', then it works
  fine. In systemd versions prior to v251, if a udev event was triggered
  that couldn't be processed because the disk's file-lock was held by
  another process, then the udev event was simply dropped. This would
  allow the udev event queue to clear. Which in turn would allow the
  'udevadm settle' call to complete.

  However, in systemd v251 a change was made to the udev event queue
  where instead of dropping udev events that are blocked by a disk's
  file-lock, the udev events are instead requeued in a loop. This
  prevents the udev event queue from clearing. Which in turn blocks the
  'udevadm settle' call in 'parted'.

  https://github.com/systemd/systemd/commit/5d354e525a56955ae7f68062e283dda85ab07794

  Systemd's official recommendation is that users should use 'flock'
  when making changes to partitions. If everyone did that, then I
  suspect the 'udevadm-settle.patch' patch would not be needed.

    https://systemd.io/BLOCK_DEVICE_LOCKING/

  However, I realize that removing the 'udevadm-settle.patch' patch
  might be undesirable since it might break back-compat for users that
  don't use 'flock' with 'parted'.

  As a workaround, the 'udevadm-settle.patch' patch could be modified to
  first check if the disk file-lock is held and if it is, assume the
  user is using 'flock' and don't call 'udevadm settle'. This should
  hopefully fix the issue for users who use 'flock', while maintaining
  existing behavior for users who don't use 'flock'.

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