kernel BUG when mounting btrfs filesystem from read-only (R/O) media

[shiftyphil]

Inserted an SD card that contained a btrfs filesystem and tried to open using nautilus.

ProblemType: KernelOops
DistroRelease: Ubuntu 11.10
Package: linux-image-3.0.0-7-generic 3.0.0-7.8
ProcVersionSignature: Ubuntu 3.0.0-7.8-generic 3.0.0
Uname: Linux 3.0.0-7-generic x86_64
AlsaVersion: Advanced Linux Sound Architecture Driver Version 1.0.24.
Annotation: Your system might become unstable now and might need to be restarted.
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC1: phil 4897 F.... pulseaudio
 /dev/snd/seq: timidity 4688 F.... timidity
CRDA: Error: [Errno 2] No such file or directory
Card0.Amixer.info:
 Card hw:0 'Intel'/'HDA Intel at 0xfc300000 irq 47'
   Mixer name : 'Realtek ALC268'
   Components : 'HDA:10ec0268,1025011f,00100003 HDA:14f12c06,10250093,00100000'
   Controls : 19
   Simple ctrls : 13
Card1.Amixer.info:
 Card hw:1 'Set'/'C-Media USB Headphone Set at usb-0000:00:1d.7-3.6.4, full speed'
   Mixer name : 'USB Mixer'
   Components : 'USB0d8c:000c'
   Controls : 7
   Simple ctrls : 3
Date: Wed Jul 27 12:13:55 2011
Failure: oops
HibernationDevice: RESUME=UUID=4e603944-5cd0-47a1-b5e4-b7b1b520d080
MachineType: Acer TravelMate 7720
PccardctlIdent:
 Socket 0:
   no product info available
PccardctlStatus:
 Socket 0:
   no card
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-3.0.0-7-generic root=/dev/mapper/vgsystem-root ro rootfstype=ext4 quiet splash vt.handoff=7
RelatedPackageVersions: kerneloops-daemon 0.12+git20090217-1ubuntu13
SourcePackage: linux
Title: ] ------------[ cut here ]------------
UpgradeStatus: Upgraded to oneiric on 2011-07-17 (10 days ago)
dmi.bios.date: 07/03/2008
dmi.bios.vendor: Phoenix Technologies LTD
dmi.bios.version: V1.35
dmi.board.name: Tangiz
dmi.board.vendor: Acer
dmi.board.version: Rev
dmi.chassis.asset.tag: None
dmi.chassis.type: 10
dmi.chassis.vendor: Acer
dmi.chassis.version: N/A
dmi.modalias: dmi:bvnPhoenixTechnologiesLTD:bvrV1.35:bd07/03/2008:svnAcer:pnTravelMate7720:pvr0100:rvnAcer:rnTangiz:rvrRev:cvnAcer:ct10:cvrN/A:
dmi.product.name: TravelMate 7720
dmi.product.version: 0100
dmi.sys.vendor: Acer

[Andy Whitcroft (apw)]

Reproduced this by formatting an SD card as btrfs, removing and enabling the lock, and reinserting. This appears to be triggered when the SD card is physically write-protected using the lock slider.

[Andy Whitcroft (apw)]

Preliminary patches sent upstream for review.

[Lapse of Reason (lapseofreason0)]

Here is a link to the upstream patch, in case anybody wants to try it out:

http://thread.gmane.org/gmane.comp.file-systems.btrfs/12319

A quick test reveals that it does fix this bug, I haven't done any further testing though.

Be careful when trying out patches on filesystem drivers, but for anyone willing to take the risk here is a quick rundown of how to rebuild the btrfs kernel module:

- unpack the kernel source (from the kernel-source package) and copy the header files (from the kernel-headers package) into the same tree
- apply the patch or make any other changes you want
- run make M=fs/btrfs (works for any other module directory too)
- copy the compiled module to /lib/modules/$VERSION/updates/
- run depmod
- reload the module

[Lapse of Reason (lapseofreason0)]

The patch I mentioned in the previous post is now in the default kernel for Ubuntu 12.04, which fixes the problem for me. Unless anyone experiences any more problems I think this bug can be marked as resolved.

[Andy Whitcroft (apw)]

This was fixed by the commit below which was commited in v3.2-rc4, this is therefore fixed for precise and later at the very least:

  commit b772a86ea6d932ac29d5e50e67c977653c832f8a
  Author: Li Zefan <email address hidden>
  Date: Mon Nov 28 16:43:00 2011 +0800

    Btrfs: fix oops when calling statfs on readonly device

Closing out fixed.