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[Thomas Mayer <1559723@xxxxxxxxxxxxxxxxxx>]

As described in my previous comment, the fs was available in read-write
mode again.

I added another disk to the btrfs and with the additional space I was
able to rerun and successfully finish the btrfs balance operation (as
described in https://www.slicewise.net/debian/balancierung-eines-vollen-
btrfs-dateisystems/ ):

btrfs balance start x
Done, had to relocate 2797 out of 2797 chunks

So my file system is completely rescued now.

To sum it up:

There is no way I could reproduce this error again with a recent kernel.

All I can say is that Kernel 4.2.0-27.32~14.04.1-generic 4.2.8-ckt1 ran
into unexpected behaviour when doing a btrfs balance:

Expected Result: Error "enospc errors during balance" (at least)

Actual Result: Kernel bug in dmesg, Filesystem goes read-only,
Input/Output errors occur, many directories are not readable even in
read-only mode, even ls does not work. It seems as if the system totally
screwed it up.

Suggested Fix: All of these problems should be avoidable because the
later reboot "healed" them (Error "enospc errors during balance"
reported, fs mounted read-write, but btrfs balance still fails, which is
totally fine at this point)

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https://bugs.launchpad.net/bugs/1559723

Title:
  soft lockup during btrfs balance

Status in linux package in Ubuntu:
  Incomplete

Bug description:
  I performed a btrfs balance for a disk which ran out of space. When I
  did this some months ago, I got an error message, but this time, I got
  a soft lockup and the file system got mounted read-only:

  # cat /proc/version_signature
  Ubuntu 4.2.0-27.32~14.04.1-generic 4.2.8-ckt1

  I see multiple errors in dmesg output, like

  [3099814.088753] NMI watchdog: BUG: soft lockup - CPU#3 stuck for 22s! [btrfs:27516]
  [3099814.089403] Modules linked in: ufs qnx4 hfsplus hfs minix ntfs msdos jfs xfs libcrc32c intel_rapl w83627ehf x86_pkg_temp_thermal intel_powerclamp hwmon_vid coretemp kvm_intel kvm snd_hda_codec_realtek snd_hda
  _codec_generic lp joydev input_leds ppdev gpio_ich 8250_fintek parport_pc parport nuvoton_cir rc_core mac_hid snd_hda_intel snd_hda_codec snd_hda_core snd_hwdep snd_pcm snd_timer snd lpc_ich mei_me soundcore mei s
  hpchp serio_raw btrfs drbg ansi_cprng dm_crypt raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq raid0 multipath linear raid1 crct10dif_pclmul crc32_pclmul hid_generic i915 aes
  ni_intel ahci aes_x86_64 lrw usbhid video gf128mul i2c_algo_bit glue_helper e1000e ablk_helper drm_kms_helper libahci ptp cryptd hid psmouse pps_core drm pata_acpi
  [3099814.089436] CPU: 3 PID: 27516 Comm: btrfs Tainted: G     U  W    L  4.2.0-27-generic #32~14.04.1-Ubuntu
  [3099814.089437] Hardware name:                  /DQ67OW, BIOS SWQ6710H.86A.0067.2014.0313.1347 03/13/2014
  [3099814.089438] task: ffff8800aca0a940 ti: ffff880110c5c000 task.ti: ffff880110c5c000
  [3099814.089438] RIP: 0010:[<ffffffffc031f9d1>]  [<ffffffffc031f9d1>] __del_reloc_root+0xd1/0x100 [btrfs]
  [3099814.089451] RSP: 0018:ffff880110c5fa60  EFLAGS: 00000246
  [3099814.089452] RAX: ffff880142cd36f8 RBX: ffffffffc02a912a RCX: 00000000ffffffe2
  [3099814.089453] RDX: 000007ba9c902000 RSI: ffffffffc034e280 RDI: ffff88002a9e8d70
  [3099814.089454] RBP: ffff880110c5fa78 R08: 0000000000000000 R09: 00000000ffffffe2
  [3099814.089454] R10: ffffffffc02a912a R11: ffffea00029bc9c0 R12: 00000000ffffffe2
  [3099814.089455] R13: ffff880110c5fa78 R14: 000000002b4bf801 R15: ffff88042b4bf800
  [3099814.089456] FS:  00007f93651f0880(0000) GS:ffff88043e2c0000(0000) knlGS:0000000000000000
  [3099814.089457] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [3099814.089458] CR2: 0000559c1ffff7d0 CR3: 00000002fca04000 CR4: 00000000000406e0
  [3099814.089459] Stack:
  [3099814.089460]  ffff88002a9e8800 ffff88042b52b000 ffff880110c5fab8 ffff880110c5fa98
  [3099814.089461]  ffffffffc031fa25 ffff88042b4bfbc0 ffff88042b4bfbc0 ffff880110c5faf8
  [3099814.089462]  ffffffffc0325f2a ffff88002a9e8d78 000000002a9e8d78 ffff88042b4bdbc0
  [3099814.089464] Call Trace:
  [3099814.089471]  [<ffffffffc031fa25>] free_reloc_roots+0x25/0x40 [btrfs]
  [3099814.089477]  [<ffffffffc0325f2a>] merge_reloc_roots+0x16a/0x230 [btrfs]
  [3099814.089483]  [<ffffffffc0326246>] relocate_block_group+0x256/0x600 [btrfs]
  [3099814.089489]  [<ffffffffc03267b3>] btrfs_relocate_block_group+0x1c3/0x2d0 [btrfs]
  [3099814.089496]  [<ffffffffc02f9dee>] btrfs_relocate_chunk.isra.39+0x3e/0xc0 [btrfs]
  [3099814.089502]  [<ffffffffc02fb26f>] __btrfs_balance+0x48f/0x8c0 [btrfs]
  [3099814.089509]  [<ffffffffc02fba1d>] btrfs_balance+0x37d/0x650 [btrfs]
  [3099814.089516]  [<ffffffffc03085e2>] ? btrfs_ioctl_balance+0x282/0x530 [btrfs]
  [3099814.089522]  [<ffffffffc0308756>] btrfs_ioctl_balance+0x3f6/0x530 [btrfs]
  [3099814.089528]  [<ffffffffc030a17f>] btrfs_ioctl+0x56f/0x2470 [btrfs]
  [3099814.089532]  [<ffffffff8118559b>] ? lru_cache_add_active_or_unevictable+0x2b/0xa0
  [3099814.089534]  [<ffffffff811a5e55>] ? handle_mm_fault+0xba5/0x1840
  [3099814.089536]  [<ffffffff811fdfbd>] do_vfs_ioctl+0x2cd/0x4b0
  [3099814.089539]  [<ffffffff810642a6>] ? __do_page_fault+0x1b6/0x430
  [3099814.089541]  [<ffffffff811fe219>] SyS_ioctl+0x79/0x90
  [3099814.089543]  [<ffffffff817bc3b2>] entry_SYSCALL_64_fastpath+0x16/0x75
  [3099814.089543] Code: f0 01 00 00 49 89 84 24 c0 03 00 00 49 89 84 24 c8 03 00 00 48 81 c7 38 09 00 00 c6 07 00 66 66 66 90 48 89 df e8 b0 d3 ea c0 5b <41> 5c 41 5d 41 5e 5d c3 76 09 48 8b 5b 08 e9 5a ff ff ff 49 8d 

  I also can't access some of the directories now (even in read-only
  mode):

  cd somedir
  -su: cd: somedir: Eingabe-/Ausgabefehler

  # lsb_release -rd
  Description:    Ubuntu 14.04.4 LTS
  Release:        14.04

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