additionnal stack traces, not exactly the same as the OP ones.

Jan 21 06:05:20 filer2 kernel: [398213.798328] INFO: task nfsd:1440 blocked for more than 120 seconds.
Jan 21 06:05:20 filer2 kernel: [398213.798371]       Not tainted 3.13.0-24-generic #47-Ubuntu
Jan 21 06:05:20 filer2 kernel: [398213.798435] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Jan 21 06:05:20 filer2 kernel: [398213.798627] nfsd            D ffff88013fd14440     0  1440      2 0x00000000
Jan 21 06:05:20 filer2 kernel: [398213.798630]  ffff8800b595fce0 0000000000000002 ffff8800b921afe0 ffff8800b595ffd8
Jan 21 06:05:20 filer2 kernel: [398213.798633]  0000000000014440 0000000000014440 ffff8800b921afe0 ffffffffa01a90a0
Jan 21 06:05:20 filer2 kernel: [398213.798635]  ffffffffa01a90a4 ffff8800b921afe0 00000000ffffffff ffffffffa01a90a8
Jan 21 06:05:20 filer2 kernel: [398213.798638] Call Trace:
Jan 21 06:05:20 filer2 kernel: [398213.798642]  [<ffffffff8171a409>] schedule_preempt_disabled+0x29/0x70
Jan 21 06:05:20 filer2 kernel: [398213.798645]  [<ffffffff8171c275>] __mutex_lock_slowpath+0x135/0x1b0
Jan 21 06:05:20 filer2 kernel: [398213.798654]  [<ffffffffa00d09b0>] ? svcauth_unix_domain_release+0x30/0x30 [sunrpc]
Jan 21 06:05:20 filer2 kernel: [398213.798658]  [<ffffffff8171c30f>] mutex_lock+0x1f/0x2f
Jan 21 06:05:20 filer2 kernel: [398213.798665]  [<ffffffffa019714f>] nfsd4_renew+0x5f/0xf0 [nfsd]
Jan 21 06:05:20 filer2 kernel: [398213.798672]  [<ffffffffa01865da>] nfsd4_proc_compound+0x56a/0x7b0 [nfsd]
Jan 21 06:05:20 filer2 kernel: [398213.798677]  [<ffffffffa0172d2b>] nfsd_dispatch+0xbb/0x200 [nfsd]
Jan 21 06:05:20 filer2 kernel: [398213.798686]  [<ffffffffa00cd63d>] svc_process_common+0x46d/0x6d0 [sunrpc]
Jan 21 06:05:20 filer2 kernel: [398213.798696]  [<ffffffffa00cd9a7>] svc_process+0x107/0x170 [sunrpc]
Jan 21 06:05:20 filer2 kernel: [398213.798701]  [<ffffffffa017271f>] nfsd+0xbf/0x130 [nfsd]
Jan 21 06:05:20 filer2 kernel: [398213.798706]  [<ffffffffa0172660>] ? nfsd_destroy+0x80/0x80 [nfsd]
Jan 21 06:05:20 filer2 kernel: [398213.798708]  [<ffffffff8108b312>] kthread+0xd2/0xf0
Jan 21 06:05:20 filer2 kernel: [398213.798711]  [<ffffffff8108b240>] ? kthread_create_on_node+0x1d0/0x1d0
Jan 21 06:05:20 filer2 kernel: [398213.798714]  [<ffffffff817263fc>] ret_from_fork+0x7c/0xb0
Jan 21 06:05:20 filer2 kernel: [398213.798717]  [<ffffffff8108b240>] ? kthread_create_on_node+0x1d0/0x1d0
Jan 21 06:05:20 filer2 kernel: [398213.798721] INFO: task kworker/u4:2:9594 blocked for more than 120 seconds.
Jan 21 06:05:20 filer2 kernel: [398213.798882]       Not tainted 3.13.0-24-generic #47-Ubuntu
Jan 21 06:05:20 filer2 kernel: [398213.799052] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Jan 21 06:05:20 filer2 kernel: [398213.799197] kworker/u4:2    D ffff88013fd14440     0  9594      2 0x00000000
Jan 21 06:05:20 filer2 kernel: [398213.799207] Workqueue: nfsd4 laundromat_main [nfsd]
Jan 21 06:05:20 filer2 kernel: [398213.799209]  ffff8800365cbd40 0000000000000002 ffff8800b582c7d0 ffff8800365cbfd8
Jan 21 06:05:20 filer2 kernel: [398213.799212]  0000000000014440 0000000000014440 ffff8800b582c7d0 ffffffffa01a90a0
Jan 21 06:05:20 filer2 kernel: [398213.799214]  ffffffffa01a90a4 ffff8800b582c7d0 00000000ffffffff ffffffffa01a90a8
Jan 21 06:05:20 filer2 kernel: [398213.799217] Call Trace:
Jan 21 06:05:20 filer2 kernel: [398213.799221]  [<ffffffff8171a409>] schedule_preempt_disabled+0x29/0x70
Jan 21 06:05:20 filer2 kernel: [398213.799224]  [<ffffffff8171c275>] __mutex_lock_slowpath+0x135/0x1b0
Jan 21 06:05:20 filer2 kernel: [398213.799227]  [<ffffffff8171c30f>] mutex_lock+0x1f/0x2f
Jan 21 06:05:20 filer2 kernel: [398213.799234]  [<ffffffffa0195a46>] laundromat_main+0x46/0x420 [nfsd]
Jan 21 06:05:20 filer2 kernel: [398213.799238]  [<ffffffff810838a2>] process_one_work+0x182/0x450
Jan 21 06:05:20 filer2 kernel: [398213.799252]  [<ffffffff81084641>] worker_thread+0x121/0x410
Jan 21 06:05:20 filer2 kernel: [398213.799256]  [<ffffffff81084520>] ? rescuer_thread+0x3e0/0x3e0
Jan 21 06:05:20 filer2 kernel: [398213.799258]  [<ffffffff8108b312>] kthread+0xd2/0xf0
Jan 21 06:05:20 filer2 kernel: [398213.799261]  [<ffffffff8108b240>] ? kthread_create_on_node+0x1d0/0x1d0
Jan 21 06:05:20 filer2 kernel: [398213.799264]  [<ffffffff817263fc>] ret_from_fork+0x7c/0xb0
Jan 21 06:05:20 filer2 kernel: [398213.799267]  [<ffffffff8108b240>] ? kthread_create_on_node+0x1d0/0x1d0