[Bug 1088081] Re: udev rules make it impossible to deactivate lvm volume group with vgchange -an

[Dmitrijs Ledkovs]

Interesting. Precise -> Quantal saw a very big lvm2 update. There were
changes done to udev handling and maybe some of it needs to be reverted.
It's interesting that dmsetup remove works, and maybe lvchange should
act more like dmsetup does.

** Changed in: lvm2 (Ubuntu)
     Assignee: (unassigned) => Dmitrijs Ledkovs (xnox)

** Changed in: lvm2 (Ubuntu)
   Importance: Undecided => High

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Title:
  udev rules make it impossible to deactivate lvm volume group with
  vgchange -an

Status in “lvm2” package in Ubuntu:
  Confirmed
Status in “udev” package in Ubuntu:
  Confirmed

Bug description:
  Running 'vgchange -a n volume_group_name' generates udev events that are matched by /lib/udev/rules.d/85-lvm2.rules, causing it to run 'vgchange -a y'.  This defeats the initial 'vgchange -a n' and makes it impossible to:
    * run 'cryptsetup luksClose' on the underlying encrypted device
    * safely remove a removable drive containing an LVM physical volume
    * safely use dd to copy the LVM partition to another device (LVM might be writing data)

  I turned up logging and it looks like the following happened to the instance I was watching:
    1. 'vgchange -a n vgname' caused LVM to close /dev/dm-5 (the LUKS dm device holding the LVM physical volume)
    2. udev logged this:  device /dev/dm-5 closed, synthesising 'change'
    3. a new 'change' 'block' udev event was enqueued for the LUKS dm device
    4. udev started processing the new 'change' event
    5. the event matched /lib/udev/rules.d/60-persistent-storage-dm.rules:16 so blkid was run and set ID_FS_TYPE=LVM2_member
    6. all of the conditions for /lib/udev/rules.d/85-lvm2.rules matched, so 'vgchange -a y' was run

  Brainstorming ideas for fixing this:
    * I'm not sure why udev synthesizes a change event when a dm device is closed, but disabling that bit of code should fix this bug (and probably cause many worse bugs).
    * Maybe the lvm udev rule should condition on ACTION=="add" instead of ACTION="add|change" (just tried this and unfortunately it doesn't work -- unlocking a LUKS device causes two back-to-back udev events:  one 'add' event that only appears to match /lib/udev/rules.d/50-udev-default.rules:67 and a second 'change' event that matches much more).

  Other info:
  $ lsb_release -rdc
  Description:    Ubuntu 12.10
  Release:        12.10
  Codename:       quantal

  lvm2 version: 2.02.95-4ubuntu1
  udev version: 175-0ubuntu13

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