Have same trouble, seems to be related to TCP network connection break:

May 26 10:08:57 station8 kernel: [ 6840.142121] INFO: task xfce4-panel:1620 blocked for more than 120 seconds.
May 26 10:08:57 station8 kernel: [ 6840.142143]       Not tainted 4.4.0-78-generic #99-Ubuntu
May 26 10:08:57 station8 kernel: [ 6840.142153] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
May 26 10:08:57 station8 kernel: [ 6840.142164] xfce4-panel     D ffff8802744239e8     0  1620   1448 0x00000000
May 26 10:08:57 station8 kernel: [ 6840.142189]  ffff8802744239e8 ffff8802744239d0 ffff880276661980 ffff880270a3e600
May 26 10:08:57 station8 kernel: [ 6840.142209]  ffff880274424000 ffff88027540bc24 ffff880270a3e600 00000000ffffffff
May 26 10:08:57 station8 kernel: [ 6840.142228]  ffff88027540bc28 ffff880274423a00 ffffffff8183c8f5 ffff88027540bc20
May 26 10:08:57 station8 kernel: [ 6840.142247] Call Trace:
May 26 10:08:57 station8 kernel: [ 6840.142278]  [<ffffffff8183c8f5>] schedule+0x35/0x80
May 26 10:08:57 station8 kernel: [ 6840.142297]  [<ffffffff8183cb9e>] schedule_preempt_disabled+0xe/0x10
May 26 10:08:57 station8 kernel: [ 6840.142313]  [<ffffffff8183e7d9>] __mutex_lock_slowpath+0xb9/0x130
May 26 10:08:57 station8 kernel: [ 6840.142334]  [<ffffffff8183e86f>] mutex_lock+0x1f/0x30
May 26 10:08:57 station8 kernel: [ 6840.142449]  [<ffffffffc06a170d>] cifs_reconnect_tcon+0x9d/0x340 [cifs]
May 26 10:08:57 station8 kernel: [ 6840.142553]  [<ffffffffc06a1a5a>] smb_init+0x2a/0x50 [cifs]
May 26 10:08:57 station8 kernel: [ 6840.142657]  [<ffffffffc06a8c53>] CIFSSMBQPathInfo+0x63/0x2e0 [cifs]
May 26 10:08:57 station8 kernel: [ 6840.142775]  [<ffffffffc06d420f>] cifs_query_path_info+0x6f/0x1a0 [cifs]
May 26 10:08:57 station8 kernel: [ 6840.142796]  [<ffffffff8121a017>] ? lookup_fast+0x57/0x330
May 26 10:08:57 station8 kernel: [ 6840.142816]  [<ffffffff811ed343>] ? kmem_cache_alloc_trace+0x183/0x1f0
May 26 10:08:57 station8 kernel: [ 6840.142832]  [<ffffffff8121a144>] ? lookup_fast+0x184/0x330
May 26 10:08:57 station8 kernel: [ 6840.142945]  [<ffffffffc06c2e80>] cifs_get_inode_info+0x390/0x8f0 [cifs]
May 26 10:08:57 station8 kernel: [ 6840.142963]  [<ffffffff81219f31>] ? unlazy_walk+0xc1/0x150
May 26 10:08:57 station8 kernel: [ 6840.142980]  [<ffffffff811ee9d9>] ? __kmalloc+0x1b9/0x250
May 26 10:08:57 station8 kernel: [ 6840.143090]  [<ffffffffc06b5dd2>] ? build_path_from_dentry+0xe2/0x3e0 [cifs]
May 26 10:08:57 station8 kernel: [ 6840.143201]  [<ffffffffc06b5dd2>] ? build_path_from_dentry+0xe2/0x3e0 [cifs]
May 26 10:08:57 station8 kernel: [ 6840.143315]  [<ffffffffc06c5425>] cifs_revalidate_dentry_attr+0x1d5/0x250 [cifs]
May 26 10:08:57 station8 kernel: [ 6840.143332]  [<ffffffff811ed5c7>] ? kmem_cache_alloc+0x187/0x1f0
May 26 10:08:57 station8 kernel: [ 6840.143446]  [<ffffffffc06c5551>] cifs_getattr+0x51/0x110 [cifs]
May 26 10:08:57 station8 kernel: [ 6840.143463]  [<ffffffff81213c0c>] vfs_getattr_nosec+0x2c/0x40
May 26 10:08:57 station8 kernel: [ 6840.143477]  [<ffffffff81213e26>] vfs_getattr+0x26/0x30
May 26 10:08:57 station8 kernel: [ 6840.143492]  [<ffffffff81213f08>] vfs_fstatat+0x78/0xc0
May 26 10:08:57 station8 kernel: [ 6840.143508]  [<ffffffff812144b1>] SYSC_newlstat+0x31/0x60
May 26 10:08:57 station8 kernel: [ 6840.143526]  [<ffffffff81003226>] ? exit_to_usermode_loop+0xa6/0xd0
May 26 10:08:57 station8 kernel: [ 6840.143542]  [<ffffffff812145ee>] SyS_newlstat+0xe/0x10
May 26 10:08:57 station8 kernel: [ 6840.143560]  [<ffffffff81840a32>] entry_SYSCALL_64_fastpath+0x16/0x71


uname -a
Linux station8 4.4.0-78-generic #99-Ubuntu SMP Thu Apr 27 15:29:09 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux