Same problem for me
Ubuntu version: Ubuntu 14.04.2 LTS
Kernel version: 3.13.0-53-generic #89-Ubuntu SMP Wed May 20 10:34:39 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

Jun  8 17:28:41 server1 kernel: [ 1561.791349] INFO: task nfsd:1986 blocked for more than 120 seconds.
Jun  8 17:28:41 server1 kernel: [ 1561.791366]       Tainted: P           OX 3.13.0-53-generic #89-Ubuntu
Jun  8 17:28:41 server1 kernel: [ 1561.791384] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Jun  8 17:28:41 server1 kernel: [ 1561.791405] nfsd            D ffff88041fb93180     0  1986      2 0x00000000
Jun  8 17:28:41 server1 kernel: [ 1561.791407]  ffff8803f94d18a0 0000000000000046 ffff8800d4588000 ffff8803f94d1fd8
Jun  8 17:28:41 server1 kernel: [ 1561.791409]  0000000000013180 0000000000013180 ffff8800d4588000 ffff880256c1e8f8
Jun  8 17:28:41 server1 kernel: [ 1561.791410]  ffff880256c1e8a8 ffff880256c1e900 ffff880256c1e8d0 0000000000000000
Jun  8 17:28:41 server1 kernel: [ 1561.791412] Call Trace:
Jun  8 17:28:41 server1 kernel: [ 1561.791414]  [<ffffffff81727229>] schedule+0x29/0x70
Jun  8 17:28:41 server1 kernel: [ 1561.791419]  [<ffffffffa007eaf5>] cv_wait_common+0xe5/0x120 [spl]
Jun  8 17:28:41 server1 kernel: [ 1561.791421]  [<ffffffff810ab220>] ? prepare_to_wait_event+0x100/0x100
Jun  8 17:28:41 server1 kernel: [ 1561.791426]  [<ffffffffa007eb45>] __cv_wait+0x15/0x20 [spl]
Jun  8 17:28:41 server1 kernel: [ 1561.791437]  [<ffffffffa0139da3>] dmu_buf_hold_array_by_dnode+0x233/0x570 [zfs]
Jun  8 17:28:41 server1 kernel: [ 1561.791449]  [<ffffffffa013a1bd>] dmu_buf_hold_array+0x5d/0x80 [zfs]
Jun  8 17:28:41 server1 kernel: [ 1561.791461]  [<ffffffffa013ba01>] dmu_read_uio+0x41/0xe0 [zfs]
Jun  8 17:28:41 server1 kernel: [ 1561.791480]  [<ffffffffa01bafbc>] zfs_read+0x14c/0x450 [zfs]
Jun  8 17:28:41 server1 kernel: [ 1561.791492]  [<ffffffffa01385fe>] ? dmu_object_size_from_db+0x5e/0x80 [zfs]
Jun  8 17:28:41 server1 kernel: [ 1561.791511]  [<ffffffffa01d757a>] zpl_aio_read+0xda/0x130 [zfs]
Jun  8 17:28:41 server1 kernel: [ 1561.791513]  [<ffffffff811bd9cc>] do_sync_readv_writev+0x4c/0x80
Jun  8 17:28:41 server1 kernel: [ 1561.791515]  [<ffffffff811bee90>] do_readv_writev+0xb0/0x220
Jun  8 17:28:41 server1 kernel: [ 1561.791534]  [<ffffffffa01bac57>] ? zfs_open+0x87/0x120 [zfs]
Jun  8 17:28:41 server1 kernel: [ 1561.791536]  [<ffffffff813213d3>] ? ima_get_action+0x23/0x30
Jun  8 17:28:41 server1 kernel: [ 1561.791538]  [<ffffffff813206b2>] ? process_measurement+0x82/0x2c0
Jun  8 17:28:41 server1 kernel: [ 1561.791539]  [<ffffffff811bf02d>] vfs_readv+0x2d/0x50
Jun  8 17:28:41 server1 kernel: [ 1561.791543]  [<ffffffffa0401aae>] nfsd_vfs_read.isra.12+0x6e/0x160 [nfsd]
Jun  8 17:28:41 server1 kernel: [ 1561.791547]  [<ffffffffa0402da9>] ? nfsd_open+0xb9/0x190 [nfsd]
Jun  8 17:28:41 server1 kernel: [ 1561.791551]  [<ffffffffa0403076>] nfsd_read+0x1e6/0x2c0 [nfsd]
Jun  8 17:28:41 server1 kernel: [ 1561.791557]  [<ffffffffa040ce4c>] nfsd3_proc_read+0xcc/0x170 [nfsd]
Jun  8 17:28:41 server1 kernel: [ 1561.791561]  [<ffffffffa03fdd3b>] nfsd_dispatch+0xbb/0x200 [nfsd]
Jun  8 17:28:41 server1 kernel: [ 1561.791568]  [<ffffffffa036262d>] svc_process_common+0x46d/0x6d0 [sunrpc]
Jun  8 17:28:41 server1 kernel: [ 1561.791575]  [<ffffffffa0362997>] svc_process+0x107/0x170 [sunrpc]
Jun  8 17:28:41 server1 kernel: [ 1561.791578]  [<ffffffffa03fd71f>] nfsd+0xbf/0x130 [nfsd]
Jun  8 17:28:41 server1 kernel: [ 1561.791582]  [<ffffffffa03fd660>] ? nfsd_destroy+0x80/0x80 [nfsd]
Jun  8 17:28:41 server1 kernel: [ 1561.791583]  [<ffffffff8108b6b2>] kthread+0xd2/0xf0
Jun  8 17:28:41 server1 kernel: [ 1561.791585]  [<ffffffff8108b5e0>] ? kthread_create_on_node+0x1c0/0x1c0
Jun  8 17:28:41 server1 kernel: [ 1561.791586]  [<ffffffff81733868>] ret_from_fork+0x58/0x90
Jun  8 17:28:41 server1 kernel: [ 1561.791587]  [<ffffffff8108b5e0>] ? kthread_create_on_node+0x1c0/0x1c0