/dev/zvol not created in 0.7.0 RC2

[Bronek]

Type	Version/Name
Distribution Name	Arch
Linux Kernel	4.8.17
Architecture	x64
ZFS Version	0.7.0 rc2
SPL Version	0.7.0 rc2

Following OOM panic (when testing 0.7.0 RC2) the ZFS failed to create /dev/zvol , however volumes were reported to be available by zfs list -t volume. This condition persisted even after clean restart following when problem was noticed first time. In the log I found a series of following messages, which might be related (only first and last shown below):

Jan 09 20:45:13 gdansk kernel: INFO: task z_zvol:20913 blocked for more than 120 seconds.
Jan 09 20:45:13 gdansk kernel:       Tainted: P           O    4.8.17-1-ARCH #1
Jan 09 20:45:13 gdansk kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Jan 09 20:45:13 gdansk kernel: z_zvol          D ffff8807409dbb88     0 20913      2 0x00000000
Jan 09 20:45:13 gdansk kernel:  ffff8807409dbb88 00ff8807409dbba8 ffff8810393f3800 ffff881938589c00
Jan 09 20:45:13 gdansk kernel:  0000000100000000 ffff8807409dc000 ffff881938589c00 ffffffffa073e224
Jan 09 20:45:13 gdansk kernel:  00000000ffffffff ffffffffa073e228 ffff8807409dbba0 ffffffff815c4f85
Jan 09 20:45:13 gdansk kernel: Call Trace:
Jan 09 20:45:13 gdansk kernel:  [<ffffffff815c4f85>] schedule+0x35/0x80
Jan 09 20:45:13 gdansk kernel:  [<ffffffff815c520e>] schedule_preempt_disabled+0xe/0x10
Jan 09 20:45:13 gdansk kernel:  [<ffffffff815c65e3>] __mutex_lock_slowpath+0xb3/0x120
Jan 09 20:45:13 gdansk kernel:  [<ffffffff815c666f>] mutex_lock+0x1f/0x30
Jan 09 20:45:13 gdansk kernel:  [<ffffffffa0550a59>] zvol_create_minor_impl+0x39/0x6b0 [zfs]
Jan 09 20:45:13 gdansk kernel:  [<ffffffffa0551902>] zvol_create_minors_cb+0x92/0xf0 [zfs]
Jan 09 20:45:13 gdansk kernel:  [<ffffffffa04a4882>] dmu_objset_find_impl+0x112/0x3d0 [zfs]
Jan 09 20:45:13 gdansk kernel:  [<ffffffffa0551870>] ? zvol_probe+0xb0/0xb0 [zfs]
Jan 09 20:45:13 gdansk kernel:  [<ffffffffa04e68a5>] ? spa_open_common+0x2b5/0x440 [zfs]
Jan 09 20:45:13 gdansk kernel:  [<ffffffffa0551870>] ? zvol_probe+0xb0/0xb0 [zfs]
Jan 09 20:45:13 gdansk kernel:  [<ffffffffa04a4b98>] dmu_objset_find+0x58/0x90 [zfs]
Jan 09 20:45:13 gdansk kernel:  [<ffffffffa0551651>] zvol_task_cb+0x381/0x3a0 [zfs]
Jan 09 20:45:13 gdansk kernel:  [<ffffffffa022991c>] taskq_thread+0x25c/0x460 [spl]
Jan 09 20:45:13 gdansk kernel:  [<ffffffff815c4adc>] ? __schedule+0x2fc/0x770
Jan 09 20:45:13 gdansk kernel:  [<ffffffff810a1440>] ? wake_up_q+0x80/0x80
Jan 09 20:45:13 gdansk kernel:  [<ffffffffa02296c0>] ? taskq_cancel_id+0x130/0x130 [spl]
Jan 09 20:45:13 gdansk kernel:  [<ffffffff81096a78>] kthread+0xd8/0xf0
Jan 09 20:45:13 gdansk kernel:  [<ffffffff8102b76e>] ? __switch_to+0x2ce/0x5b0
Jan 09 20:45:13 gdansk kernel:  [<ffffffff815c8cbf>] ret_from_fork+0x1f/0x40
Jan 09 20:45:13 gdansk kernel:  [<ffffffff810969a0>] ? kthread_worker_fn+0x180/0x180
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Jan 09 20:51:22 gdansk kernel: INFO: task z_zvol:20913 blocked for more than 120 seconds.
Jan 09 20:51:22 gdansk kernel:       Tainted: P           O    4.8.17-1-ARCH #1
Jan 09 20:51:22 gdansk kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Jan 09 20:51:22 gdansk kernel: z_zvol          D ffff8807409dbb88     0 20913      2 0x00000000
Jan 09 20:51:22 gdansk kernel:  ffff8807409dbb88 00ff8807409dbba8 ffff8810393f3800 ffff881938589c00
Jan 09 20:51:22 gdansk kernel:  0000000100000000 ffff8807409dc000 ffff881938589c00 ffffffffa073e224
Jan 09 20:51:22 gdansk kernel:  00000000ffffffff ffffffffa073e228 ffff8807409dbba0 ffffffff815c4f85
Jan 09 20:51:22 gdansk kernel: Call Trace:
Jan 09 20:51:22 gdansk kernel:  [<ffffffff815c4f85>] schedule+0x35/0x80
Jan 09 20:51:22 gdansk kernel:  [<ffffffff815c520e>] schedule_preempt_disabled+0xe/0x10
Jan 09 20:51:22 gdansk kernel:  [<ffffffff815c65e3>] __mutex_lock_slowpath+0xb3/0x120
Jan 09 20:51:22 gdansk kernel:  [<ffffffff815c666f>] mutex_lock+0x1f/0x30
Jan 09 20:51:22 gdansk kernel:  [<ffffffffa0550a59>] zvol_create_minor_impl+0x39/0x6b0 [zfs]
Jan 09 20:51:22 gdansk kernel:  [<ffffffffa0551902>] zvol_create_minors_cb+0x92/0xf0 [zfs]
Jan 09 20:51:22 gdansk kernel:  [<ffffffffa04a4882>] dmu_objset_find_impl+0x112/0x3d0 [zfs]
Jan 09 20:51:22 gdansk kernel:  [<ffffffffa0551870>] ? zvol_probe+0xb0/0xb0 [zfs]
Jan 09 20:51:22 gdansk kernel:  [<ffffffffa04e68a5>] ? spa_open_common+0x2b5/0x440 [zfs]
Jan 09 20:51:22 gdansk kernel:  [<ffffffffa0551870>] ? zvol_probe+0xb0/0xb0 [zfs]
Jan 09 20:51:22 gdansk kernel:  [<ffffffffa04a4b98>] dmu_objset_find+0x58/0x90 [zfs]
Jan 09 20:51:22 gdansk kernel:  [<ffffffffa0551651>] zvol_task_cb+0x381/0x3a0 [zfs]
Jan 09 20:51:22 gdansk kernel:  [<ffffffffa022991c>] taskq_thread+0x25c/0x460 [spl]
Jan 09 20:51:22 gdansk kernel:  [<ffffffff815c4adc>] ? __schedule+0x2fc/0x770
Jan 09 20:51:22 gdansk kernel:  [<ffffffff810a1440>] ? wake_up_q+0x80/0x80
Jan 09 20:51:22 gdansk kernel:  [<ffffffffa02296c0>] ? taskq_cancel_id+0x130/0x130 [spl]
Jan 09 20:51:22 gdansk kernel:  [<ffffffff81096a78>] kthread+0xd8/0xf0
Jan 09 20:51:22 gdansk kernel:  [<ffffffff8102b76e>] ? __switch_to+0x2ce/0x5b0
Jan 09 20:51:22 gdansk kernel:  [<ffffffff815c8cbf>] ret_from_fork+0x1f/0x40
Jan 09 20:51:22 gdansk kernel:  [<ffffffff810969a0>] ? kthread_worker_fn+0x180/0x180

[Bronek]

I did not see it again.