Fix a problem that hangs the build uninteruptably
Fix a problem that hangs the build uninteruptably

The problem is a bug in either the nbd kernel module or qemu-nbd.
This is the dmesg output:
[200560.607107] INFO: task qemu-nbd:5522 blocked for more than 120 seconds.
[200560.607108]       Tainted: P           O   3.18.5-1-ARCH #1
[200560.607108] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[200560.607109] qemu-nbd        D 0000000000000003     0  5522      1 0x00000000
[200560.607110]  ffff88039f0c7d90 0000000000000086 ffff88009f0b5080 0000000000013640
[200560.607112]  ffff88039f0c7fd8 0000000000013640 ffff8804295d8a10 ffff88009f0b5080
[200560.607113]  ffffffff810a2085 ffff88039f0c7f28 ffff88039f0c7d98 0000000200000000
[200560.607115] Call Trace:
[200560.607117]  [<ffffffff810a2085>] ? sched_clock_cpu+0xb5/0xe0
[200560.607118]  [<ffffffff810dbb90>] ? __run_hrtimer+0x250/0x250
[200560.607120]  [<ffffffff810dc2b4>] ? hrtimer_start_range_ns+0x14/0x20
[200560.607122]  [<ffffffff81551269>] schedule+0x29/0x70
[200560.607123]  [<ffffffff81553935>] rwsem_down_read_failed+0xe5/0x130
[200560.607125]  [<ffffffff812c0b04>] call_rwsem_down_read_failed+0x14/0x30
[200560.607126]  [<ffffffff815532a7>] ? down_read+0x17/0x20
[200560.607128]  [<ffffffff81199ba9>] SyS_madvise+0x689/0x870
[200560.607129]  [<ffffffff8109e930>] ? wake_up_process+0x50/0x50
[200560.607131]  [<ffffffff810efbf5>] ? SyS_futex+0x85/0x1a0
[200560.607133]  [<ffffffff815553a9>] system_call_fastpath+0x12/0x17

The workaround consist of allowing only 'raw' build images, which are
mounted as loopback devices and not via qemu-nbd.

sheyll/b9-vm-image-builder