> Curiously I do not see this when running the tests on my laptop

And of course in the minute I write this it does happen again (I ran the test maybe 20 times today, and only just now it happened):

[11359.569157] INFO: task systemd-udevd:341 blocked for more than 120 seconds.
[11359.569163]       Not tainted 4.8.0-29-generic #31~lp1626436ProposedWithTwoPatches
[11359.569165] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[11359.569167] systemd-udevd   D ffffa31788e77ba8     0   341      1 0x00000100
[11359.569173]  ffffa31788e77ba8 00ffffff84c71a68 ffffa31499670000 ffffa31788e82ac0
[11359.569177]  0000000088e77db0 ffffa31788e78000 ffffa317893bc070 ffffa317893bc088
[11359.569179]  ffffa31788e77be0 ffffa3149cf771c0 ffffa31788e77bc0 ffffffff85296905
[11359.569182] Call Trace:
[11359.569190]  [<ffffffff85296905>] schedule+0x35/0x80
[11359.569193]  [<ffffffff85299543>] rwsem_down_read_failed+0x103/0x160
[11359.569197]  [<ffffffff84e3eb98>] call_rwsem_down_read_failed+0x18/0x30
[11359.569200]  [<ffffffff85298da0>] down_read+0x20/0x40
[11359.569202]  [<ffffffff84c35e15>] get_super.part.7+0x95/0xd0
[11359.569204]  [<ffffffff84c35e76>] get_super+0x26/0x30
[11359.569208]  [<ffffffff84c70f98>] fsync_bdev+0x18/0x60
[11359.569211]  [<ffffffff84e0f064>] invalidate_partition+0x24/0x50
[11359.569213]  [<ffffffff84e12243>] rescan_partitions+0x53/0x2b0
[11359.569216]  [<ffffffff84d728d8>] ? security_capable+0x48/0x60
[11359.569218]  [<ffffffff84e0d0d5>] __blkdev_reread_part+0x65/0x70
[11359.569220]  [<ffffffff84e0d103>] blkdev_reread_part+0x23/0x40
[11359.569222]  [<ffffffff84e0db25>] blkdev_ioctl+0x425/0x910
[11359.569225]  [<ffffffff84b43907>] ? __seccomp_filter+0x67/0x260
[11359.569228]  [<ffffffff84c7090d>] block_ioctl+0x3d/0x50
[11359.569231]  [<ffffffff84c47883>] do_vfs_ioctl+0xa3/0x610
[11359.569233]  [<ffffffff84b441b3>] ? __secure_computing+0x43/0xe0
[11359.569236]  [<ffffffff84a033ae>] ? syscall_trace_enter+0xce/0x2f0
[11359.569239]  [<ffffffff84c47e69>] SyS_ioctl+0x79/0x90
[11359.569241]  [<ffffffff84a03b9e>] do_syscall_64+0x5e/0xc0
[11359.569244]  [<ffffffff8529b165>] entry_SYSCALL64_slow_path+0x25/0x25
[11359.569320] INFO: task mount.ntfs:15670 blocked for more than 120 seconds.
[11359.569323]       Not tainted 4.8.0-29-generic #31~lp1626436ProposedWithTwoPatches
[11359.569324] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[11359.569325] mount.ntfs      D ffffa314d0d1bd68     0 15670   5268 0x00000000
[11359.569329]  ffffa314d0d1bd68 0000000100000003 ffffa3178bef9c80 ffffa31499670000
[11359.569332]  000000003d862ecf ffffa314d0d1c000 ffffa3149cf771dc ffffa31499670000
[11359.569334]  00000000ffffffff ffffa3149cf771e0 ffffa314d0d1bd80 ffffffff85296905
[11359.569337] Call Trace:
[11359.569340]  [<ffffffff85296905>] schedule+0x35/0x80
[11359.569342]  [<ffffffff85296bae>] schedule_preempt_disabled+0xe/0x10
[11359.569344]  [<ffffffff852987e9>] __mutex_lock_slowpath+0xb9/0x130
[11359.569347]  [<ffffffff8529887f>] mutex_lock+0x1f/0x30
[11359.569350]  [<ffffffff84c71440>] blkdev_put+0x20/0x110
[11359.569352]  [<ffffffff84c71564>] blkdev_close+0x34/0x70
[11359.569354]  [<ffffffff84c34f77>] __fput+0xe7/0x230
[11359.569356]  [<ffffffff84c3512e>] ____fput+0xe/0x10
[11359.569358]  [<ffffffff84aa2160>] task_work_run+0x80/0xa0
[11359.569361]  [<ffffffff84a032d2>] exit_to_usermode_loop+0xc2/0xd0
[11359.569363]  [<ffffffff84a03b2e>] syscall_return_slowpath+0x4e/0x60
[11359.569366]  [<ffffffff8529b13e>] entry_SYSCALL_64_fastpath+0xa6/0xa8


root     15670  0.0  0.0  16040   460 ?        Ds   10:41   0:00 /sbin/mount.ntfs /dev/sdb /run/media/root/553BDB6430706196 -o rw,nodev,nosuid,uid=0,gid=0,uhelper=udisks2


(uninterruptible kernel deep sleep, 'D' state).