For no apparent reason I cannot longer mount my RAID1-pair of
500GB-disks read/write. If I do this, the kernel crashes invariably a
few seconds after the first read attempt. The syslog contains always:


  kernel: BTRFS critical (device sdc): unable to update root key 5 132 0
  kernel: ------------[ cut here ]------------
  kernel: kernel BUG at fs/btrfs/root-tree.c:160!
  kernel: invalid opcode: 0000 [#1] SMP

Fortunately I can mount the disks read-only without problems.
(So I will not try "btrfs check" before I have copied the disks.)

"btrfs scrub" works and found no errors
"btrfs rescue super-recover" works but does not help.
"mount -o recovery" does not help

This happens with all kernels I tried: 3.16, 3.16.7-ctk7, 3.18.9,
3.19.1, 4.0-rc2

Regards,

Martin

PS: I attach the relevant part of syslog.