I just got this on my laptop:

[917043.074390] kernel BUG at /build/buildd/linux-2.6.38/fs/dcache.c:947!
[917043.074468] invalid opcode: 0000 [#1] SMP 
[917043.074521] last sysfs file: /sys/devices/platform/dock.0/flags
[917043.074593] CPU 0 
[917043.074618] Modules linked in: binfmt_misc vboxnetadp vboxnetflt vboxdrv nfs lockd fscache nfs_acl auth_rpcgss sunrpc autofs4 nls_iso8859_1 nls_cp437 vfat fat usb_storage uas ip6table_filter ip6_tables ipt_MASQUERADE iptable_nat nf_nat nf_conntrack_ipv4 nf_defrag_ipv4 xt_state nf_conntrack ipt_REJECT xt_CHECKSUM iptable_mangle xt_tcpudp iptable_filter ip_tables x_tables bridge stp snd_seq_dummy rfcomm sco bnep l2cap zaurus cdc_wdm cdc_acm cdc_ether usbnet parport_pc ppdev btusb bluetooth snd_hda_codec_conexant snd_hda_intel snd_hda_codec snd_hwdep snd_pcm thinkpad_acpi snd_seq_midi arc4 usblp snd_rawmidi snd_seq_midi_event iwlagn snd_seq snd_timer snd_seq_device iwlcore snd tpm_tis uvcvideo mac80211 videodev psmouse nvram serio_raw tpm lp tpm_bios soundcore snd_page_alloc v4l2_compat_ioctl32 parport cfg80211 joydev sha256_generic cryptd aes_x86_64 aes_generic dm_crypt usbhid hid i915 drm_kms_helper ahci drm e1000e libahci i2c_algo_bit video
[917043.075920] 
[917043.075942] Pid: 30720, comm: umount.nfs Not tainted 2.6.38-8-generic #42-Ubuntu LENOVO 7459L88/7459L88
[917043.076067] RIP: 0010:[<ffffffff81179945>]  [<ffffffff81179945>] shrink_dcache_for_umount_subtree+0x285/0x290
[917043.079565] RSP: 0018:ffff88000bcc3d78  EFLAGS: 00010292
[917043.083050] RAX: 0000000000000066 RBX: ffff88009ea0b540 RCX: 000000000000009f
[917043.083357] RDX: 0000000000000000 RSI: 0000000000000082 RDI: 0000000000000246
[917043.083357] RBP: ffff88000bcc3db8 R08: 0000000000000033 R09: 000000000005e51e
[917043.083357] R10: 0000000000000000 R11: 0000000000000001 R12: ffff88009ea0b59c
[917043.083357] R13: ffff88009ea0b540 R14: ffff880015626a60 R15: ffff88009ea0b59c
[917043.083357] FS:  00007fd1c1d9a720(0000) GS:ffff8800bd200000(0000) knlGS:0000000000000000
[917043.083357] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[917043.083357] CR2: 00007fb7cc183000 CR3: 00000001002bf000 CR4: 00000000000426f0
[917043.083357] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[917043.083357] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[917043.083357] Process umount.nfs (pid: 30720, threadinfo ffff88000bcc2000, task ffff88012ed35b80)
[917043.083357] Stack:
[917043.083357]  ffff88012dac4658 ffffffff8100a777 ffff8801179ec4a0 ffff88012dac4400
[917043.083357]  ffff88009ea0ba1c ffff88009ea0b9c0 ffff880106e9e180 0000000000000000
[917043.083357]  ffff88000bcc3de8 ffffffff8117c2b9 ffff8800bd213d00 ffff88012dac4400
[917043.083357] Call Trace:
[917043.083357]  [<ffffffff8100a777>] ? __switch_to+0x157/0x2f0
[917043.083357]  [<ffffffff8117c2b9>] shrink_dcache_for_umount+0x69/0x90
[917043.083357]  [<ffffffff81166bfc>] generic_shutdown_super+0x2c/0x100
[917043.083357]  [<ffffffff81166d66>] kill_anon_super+0x16/0x60
[917043.083357]  [<ffffffffa05ce365>] nfs_kill_super+0x25/0x40 [nfs]
[917043.083357]  [<ffffffff811671d5>] deactivate_locked_super+0x45/0x70
[917043.083357]  [<ffffffff81167e5a>] deactivate_super+0x4a/0x70
[917043.083357]  [<ffffffff81183254>] mntput_no_expire+0xa4/0xf0
[917043.083357]  [<ffffffff811832bf>] mntput+0x1f/0x30
[917043.083357]  [<ffffffff81183b72>] release_mounts+0x72/0x90
[917043.083357]  [<ffffffff81184162>] do_umount+0xd2/0x1f0
[917043.083357]  [<ffffffff81184343>] sys_umount+0xc3/0xd0
[917043.083357]  [<ffffffff8100c002>] system_call_fastpath+0x16/0x1b
[917043.083357] Code: 8b 40 28 4c 8b 08 49 8b 45 30 48 85 c0 74 07 48 8b 90 a8 00 00 00 48 89 34 24 48 c7 c7 50 6c 7e 81 4c 89 ee 31 c0 e8 94 66 44 00 <0f> 0b 0f 0b 0f 1f 80 00 00 00 00 55 48 89 e5 48 83 ec 20 48 89 
[917043.083357] RIP  [<ffffffff81179945>] shrink_dcache_for_umount_subtree+0x285/0x290
[917043.083357]  RSP <ffff88000bcc3d78>


Restarting autofs fixed it. I access NFS4 volumes via autofs.