I checked the same usage scenario with -rc2 kernel..
Looks like it is just because of allmodconfig. Interesting fact,
though: now warnings are thrown on on-disk operations, errors are on
rbtree operations.
[ 781.230303] ------------[ cut here ]------------
[ 781.230368] WARNING: at fs/btrfs/inode.c:3117 inode_tree_add+0xd0/0x162
[btrfs]()
[ 781.230371] Hardware name: X51RL
[ 781.230374] Modules linked in: usblp raid456 raid6_pq async_xor async_memcpy
async_tx xor snd_pcm_oss snd_mixer_oss raid1 af_packet raid0 md_mod ipv6 ipip
tunnel4 radeon drm snd_hda_codec_si3054 snd_hda_codec_realtek snd_hda_intel
snd_hda_codec snd_hwdep snd_pcm snd_timer snd soundcore snd_page_alloc atiixp
ftdi_sio usbserial loop ati_agp agpgart p4_clockmod speedstep_lib 8139too mii
kqemu fuse thermal thermal_sys hwmon ac battery asus_laptop backlight led_class
tun usb_storage usb_libusual dm_mod ide_generic ide_gd_mod ide_cd_mod ide_core
sr_mod cdrom pata_marvell ata_piix sata_uli sata_sis pata_sis sata_via sata_nv
ahci fbdev xtkbd atkbd ohci_hcd ssb pcmcia pcmcia_core firmware_class ehci_hcd
uhci_hcd usbhid hid usbcore unix btrfs zlib_deflate libcrc32c crc32c libata
sd_mod scsi_mod crc_t10dif jfs nls_base xfs exportfs ext3 jbd mbcache
synaptics_i2c i2c_core sermouse psmouse libps2 pcips2 i8042 serio evdev mousedev
[ 781.230479] Pid: 25678, comm: perl Not tainted 2.6.31-rc2 #1
[ 781.230482] Call Trace:
[ 781.230523] [<f8c16113>] ? inode_tree_add+0xd0/0x162 [btrfs]
[ 781.230532] [<c106412d>] warn_slowpath_common+0xe0/0x148
[ 781.230538] [<c10641be>] warn_slowpath_null+0x29/0x3a
[ 781.230579] [<f8c16113>] inode_tree_add+0xd0/0x162 [btrfs]
[ 781.230621] [<f8c1c933>] btrfs_iget+0x6cb/0x6f8 [btrfs]
[ 781.230627] [<c103f829>] ? kmap_atomic+0x22/0x32
[ 781.230669] [<f8c1cbe8>] btrfs_lookup_dentry+0x288/0x2d1 [btrfs]
[ 781.230711] [<f8c1cc68>] btrfs_lookup+0x37/0x82 [btrfs]
[ 781.230728] [<c1185273>] do_lookup+0x153/0x2cb
[ 781.230733] [<c1186bb6>] __link_path_walk+0xcf8/0xf61
[ 781.230738] [<c1187951>] path_walk+0xb4/0x19b
[ 781.230743] [<c1189c33>] do_path_lookup+0x4f/0x193
[ 781.230748] [<c118c423>] user_path_at+0x94/0xe4
[ 781.230755] [<c117d819>] vfs_fstatat+0x5f/0xc0
[ 781.230761] [<c117d906>] vfs_lstat+0x24/0x34
[ 781.230766] [<c117d938>] sys_lstat64+0x22/0x60
[ 781.230772] [<c1005523>] sysenter_do_call+0x12/0x28
[ 781.230776] ---[ end trace aefd5bf8b0fdf91e ]---
