Comment 32 for bug 561210

And, indeed, if the umount process is stuck for long enough, the following kernel stack-trace is emitted:

Jul 26 21:38:49 illustrious kernel: [ 838.729063] INFO: task umount.nfs:2570 blocked for more than 120 seconds.
Jul 26 21:38:49 illustrious kernel: [ 838.729069] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Jul 26 21:38:49 illustrious kernel: [ 838.729073] umount.nfs D 0000000000000000 0 2570 1 0x00000000
Jul 26 21:38:49 illustrious kernel: [ 838.729080] ffff8801d5747d98 0000000000000086 0000000000015bc0 0000000000015bc0
Jul 26 21:38:49 illustrious kernel: [ 838.729087] ffff880210ec03c0 ffff8801d5747fd8 0000000000015bc0 ffff880210ec0000
Jul 26 21:38:49 illustrious kernel: [ 838.729092] 0000000000015bc0 ffff8801d5747fd8 0000000000015bc0 ffff880210ec03c0
Jul 26 21:38:49 illustrious kernel: [ 838.729098] Call Trace:
Jul 26 21:38:49 illustrious kernel: [ 838.729110] [<ffffffff811650d0>] ? bdi_sched_wait+0x0/0x20
Jul 26 21:38:49 illustrious kernel: [ 838.729115] [<ffffffff811650de>] bdi_sched_wait+0xe/0x20
Jul 26 21:38:49 illustrious kernel: [ 838.729123] [<ffffffff8153f3af>] __wait_on_bit+0x5f/0x90
Jul 26 21:38:49 illustrious kernel: [ 838.729127] [<ffffffff811650d0>] ? bdi_sched_wait+0x0/0x20
Jul 26 21:38:49 illustrious kernel: [ 838.729132] [<ffffffff8153f458>] out_of_line_wait_on_bit+0x78/0x90
Jul 26 21:38:49 illustrious kernel: [ 838.729140] [<ffffffff81085360>] ? wake_bit_function+0x0/0x40
Jul 26 21:38:49 illustrious kernel: [ 838.729144] [<ffffffff81165094>] ? bdi_queue_work+0xa4/0xe0
Jul 26 21:38:49 illustrious kernel: [ 838.729149] [<ffffffff8116640f>] bdi_sync_writeback+0x6f/0x80
Jul 26 21:38:49 illustrious kernel: [ 838.729154] [<ffffffff81166440>] sync_inodes_sb+0x20/0x30
Jul 26 21:38:49 illustrious kernel: [ 838.729160] [<ffffffff81169f12>] __sync_filesystem+0x82/0x90
Jul 26 21:38:49 illustrious kernel: [ 838.729164] [<ffffffff81169ff9>] sync_filesystems+0xd9/0x130
Jul 26 21:38:49 illustrious kernel: [ 838.729171] [<ffffffff8115ece1>] sys_umount+0xb1/0xd0
Jul 26 21:38:49 illustrious kernel: [ 838.729178] [<ffffffff810131b2>] system_call_fastpath+0x16/0x1b

I've also identified that my test NFS mount was using UDP, and performs several times *better* in terms of IO throughput when switched to TCP operation. This lack of performance (and responsiveness) may have been masking some other issues, so I'll use TCP for future testing.