On my main workstation at home, I have autofs set up to mount the pseudoroot of my main fileserver (details on this to follow). When I mount that root and do even a small bit of I/O to it, and then let the mount expire, I get a BUG() pop on umount:

[  523.953284] BUG: Dentry ffff8801f3084180{i=2,n=} still in use (1) [unmount of nfs4 0:2c]
[  523.953306] ------------[ cut here ]------------
[  523.954013] kernel BUG at fs/dcache.c:925!
[  523.954013] invalid opcode: 0000 [#1] SMP 
[  523.954013] last sysfs file: /sys/devices/virtual/bdi/0:45/uevent
[  523.954013] CPU 1 
[  523.954013] Modules linked in: nfs lockd auth_rpcgss nfs_acl tun fuse ip6table_filter ip6_tables ebtable_nat ebtables sunrpc cachefiles fscache cpufreq_ondemand powernow_k8 freq_table mperf it87 adt7475 hwmon_vid xfs snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_intel raid1 snd_hda_codec snd_usb_audio snd_usbmidi_lib snd_hwdep snd_seq snd_rawmidi snd_seq_device snd_pcm snd_timer snd uvcvideo ppdev videodev soundcore media sp5100_tco v4l2_compat_ioctl32 edac_core parport_pc snd_page_alloc i2c_piix4 edac_mce_amd k10temp parport wmi r8169 microcode mii virtio_net kvm_amd kvm ipv6 ata_generic pata_acpi pata_atiixp radeon ttm drm_kms_helper drm i2c_algo_bit i2c_core [last unloaded: scsi_wait_scan]
[  523.954013] 
[  523.954013] Pid: 8387, comm: umount.nfs4 Not tainted 2.6.39-1.fc16.x86_64 #1 BIOSTAR Group TA790GX 128M/TA790GX 128M
[  523.954013] RIP: 0010:[<ffffffff81131788>]  [<ffffffff81131788>] shrink_dcache_for_umount_subtree+0x104/0x1c9
[  523.954013] RSP: 0018:ffff8801f2385d98  EFLAGS: 00010296
[  523.954013] RAX: 0000000000000062 RBX: ffff8801f3084180 RCX: 00000000000027a7
[  523.954013] RDX: 0000000000000000 RSI: 0000000000000046 RDI: 0000000000000246
[  523.954013] RBP: ffff8801f2385dc8 R08: 0000000000000002 R09: 0000ffff00066c0a
[  523.954013] R10: 0000ffff00066c0a R11: 0000000000000003 R12: ffff8801f3084180
[  523.954013] R13: ffff8801f31305f0 R14: ffff8801f30ec5f0 R15: 0000000000000000
[  523.954013] FS:  00007f08e468d720(0000) GS:ffff88022fc80000(0000) knlGS:00000000f770f870
[  523.954013] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  523.954013] CR2: 00007fff8fc29ff8 CR3: 00000001f7ac4000 CR4: 00000000000006e0
[  523.954013] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  523.954013] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[  523.954013] Process umount.nfs4 (pid: 8387, threadinfo ffff8801f2384000, task ffff8801f7a61710)
[  523.954013] Stack:
[  523.954013]  ffff8801fdd74a58 0000000000000000 ffff8801fdd74800 ffff8801f3084240
[  523.954013]  ffff8801f2385f28 ffff8801f30ec540 ffff8801f2385de8 ffffffff81132063
[  523.954013]  ffff8801fdd74800 ffffffffa0542300 ffff8801f2385e18 ffffffff81121f15
[  523.954013] Call Trace:
[  523.954013]  [<ffffffff81132063>] shrink_dcache_for_umount+0x4d/0x5f
[  523.954013]  [<ffffffff81121f15>] generic_shutdown_super+0x23/0xde
[  523.954013]  [<ffffffff8112204d>] kill_anon_super+0x13/0x4c
[  523.954013]  [<ffffffffa051c436>] nfs4_kill_super+0x44/0x77 [nfs]
[  523.954013]  [<ffffffff81122271>] deactivate_locked_super+0x26/0x4b
[  523.954013]  [<ffffffff81122a83>] deactivate_super+0x37/0x3b
[  523.954013]  [<ffffffff811375b3>] mntput_no_expire+0xcc/0xd1
[  523.954013]  [<ffffffff811375de>] mntput+0x26/0x28
[  523.954013]  [<ffffffff81137bef>] release_mounts+0x59/0x73
[  523.954013]  [<ffffffff81138155>] sys_umount+0x28d/0x2da
[  523.954013]  [<ffffffff814828c2>] system_call_fastpath+0x16/0x1b
[  523.954013] Code: 8b 40 28 4c 8b 08 48 8b 43 30 48 85 c0 74 07 48 8b 90 a8 00 00 00 48 89 34 24 48 c7 c7 f9 ed 7a 81 48 89 de 31 c0 e8 4e 17 34 00 <0f> 0b 4c 8b 63 18 4c 39 e3 75 11 48 8d bb 90 00 00 00 45 31 e4 
[  523.954013] RIP  [<ffffffff81131788>] shrink_dcache_for_umount_subtree+0x104/0x1c9
[  523.954013]  RSP <ffff8801f2385d98>