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[Tomas Pospisek <tpo_deb@xxxxxxxxxxxxx>]

So I'm reporting my current further findings here in case somebody else
than me stumbles upon this bug report in search of help.

I'll not create a new bug report as of now, since I'm still researching
the problem and I think there's no point in opening a new bug report if
it'll turn out to be invalid anyway.

So to the point: in my case the problem has been:

  8 05:02:51 foo kernel: [987432.148305] device-mapper: snapshots:
Invalidating snapshot: Unable to allocate exception.

after that umount goes berzerk:

  Nov  8 05:02:51 foo kernel: [987432.163602] Buffer I/O error on device dm-11, logical block 11985789
  Nov  8 05:02:51 foo kernel: [987432.163638] Buffer I/O error on device dm-11, logical block 11985790
  ...
  Nov  8 05:02:51 foo kernel: [987432.299393] EXT4-fs warning (device loop0): __ext4_read_dirblock:901: error reading directory block (ino 3539872, block 0)
  Nov  8 05:02:57 foo kernel: [987437.577541] quiet_error: 4396 callbacks suppressed
  Nov  8 05:02:57 foo kernel: [987437.577547] Buffer I/O error on device dm-11, logical block 12622895
  Nov  8 05:02:57 foo kernel: [987437.577609] lost page write due to I/O error on dm-11

and now umount is hanging there with 100% CPU usage with no possibility
to `kill` it.

So one problem here is that the `lvsnapshot` is running out of
`exception` space. Thus I should either allow the snapshot to be
autoextended or to increase the `-L`size I have given it.

But even so, umount shouldn't get stuck on the ext4 FS. Since I was
`umount`ing without `-f` I will try that today and see what happens and
report back here hopefully.

-- 
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https://bugs.launchpad.net/bugs/1115753

Title:
  Umount of Multiple LVM Snapshots Causes  'soft lockup CPU#0 stuck for'

Status in linux package in Ubuntu:
  Invalid

Bug description:
  Simultaneous umount of multiple lvm snapshots causes system to hang /
  freeze / deadlock.  Window manager freezes.  If umount is executed in
  a non window-manager tty,  messages containing "BUG", "blocked for
  more than xx seconds", and a list of tasks associated with each of the
  8 cpu cores will scroll up at regular intervals.  The tasks assigned
  to each cpu core are unchanging over time, appearing to be deadlocked.
  No relevant information is left in system logs.  Hard reset / poweroff
  is required.  Problem does not occur 100% of the time.  It is less
  likely to occur if umount is done shortly after mount.  Problem still
  occurs -- but less frequently -- if lazy umount -l is used.  Snapshots
  are read-only, and are mounted read-only.

  ProblemType: Bug
  DistroRelease: Ubuntu 12.04
  Package: linux-image-3.2.0-37-generic 3.2.0-37.58
  ProcVersionSignature: Ubuntu 3.2.0-37.58-generic 3.2.35
  Uname: Linux 3.2.0-37-generic x86_64
  AlsaVersion: Advanced Linux Sound Architecture Driver Version 1.0.24.
  ApportVersion: 2.0.1-0ubuntu17.1
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC1:  craig      3257 F.... pulseaudio
   /dev/snd/controlC0:  craig      3257 F.... pulseaudio
  CRDA: Error: command ['iw', 'reg', 'get'] failed with exit code 1: nl80211 not found.
  Card0.Amixer.info:
   Card hw:0 'SB'/'HDA ATI SB at 0xfeb00000 irq 16'
     Mixer name	: 'Realtek ALC889'
     Components	: 'HDA:10ec0889,1043846b,00100004'
     Controls      : 49
     Simple ctrls  : 24
  Card1.Amixer.info:
   Card hw:1 'HDMI'/'HDA ATI HDMI at 0xfea30000 irq 98'
     Mixer name	: 'ATI R6xx HDMI'
     Components	: 'HDA:1002aa01,00aa0100,00100100'
     Controls      : 6
     Simple ctrls  : 1
  Card1.Amixer.values:
   Simple mixer control 'IEC958',0
     Capabilities: pswitch pswitch-joined penum
     Playback channels: Mono
     Mono: Playback [on]
  Date: Mon Feb  4 18:58:37 2013
  HibernationDevice: RESUME=UUID=b788f786-ba76-4eb0-991f-d3bbc339d3f7
  InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Release amd64 (20111011)
  IwConfig:
   lo        no wireless extensions.
   
   eth0      no wireless extensions.
  MachineType: To be filled by O.E.M. To be filled by O.E.M.
  MarkForUpload: True
  ProcEnviron:
   TERM=xterm
   PATH=(custom, no user)
   LANG=en_US.UTF-8
   SHELL=/bin/bash
  ProcFB: 0 VESA VGA
  ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-3.2.0-37-generic root=/dev/mapper/volgroup0-root ro
  RelatedPackageVersions:
   linux-restricted-modules-3.2.0-37-generic N/A
   linux-backports-modules-3.2.0-37-generic  N/A
   linux-firmware                            1.79.1
  RfKill:
   
  SourcePackage: linux
  UpgradeStatus: Upgraded to precise on 2012-10-23 (104 days ago)
  dmi.bios.date: 09/27/2011
  dmi.bios.vendor: American Megatrends Inc.
  dmi.bios.version: 0813
  dmi.board.asset.tag: To be filled by O.E.M.
  dmi.board.name: Crosshair V Formula
  dmi.board.vendor: ASUSTeK Computer INC.
  dmi.board.version: Rev 1.xx
  dmi.chassis.asset.tag: To Be Filled By O.E.M.
  dmi.chassis.type: 3
  dmi.chassis.vendor: To Be Filled By O.E.M.
  dmi.chassis.version: To Be Filled By O.E.M.
  dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr0813:bd09/27/2011:svnTobefilledbyO.E.M.:pnTobefilledbyO.E.M.:pvrTobefilledbyO.E.M.:rvnASUSTeKComputerINC.:rnCrosshairVFormula:rvrRev1.xx:cvnToBeFilledByO.E.M.:ct3:cvrToBeFilledByO.E.M.:
  dmi.product.name: To be filled by O.E.M.
  dmi.product.version: To be filled by O.E.M.
  dmi.sys.vendor: To be filled by O.E.M.

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