Comment 21 for bug 1398859

strace of udevd while it's hanging:

10134 open("/dev/sda", O_RDONLY|O_NONBLOCK|O_NOFOLLOW|O_CLOEXEC) = 11
10134 flock(11, LOCK_EX|LOCK_NB) = 0
10134 ioctl(11, BLKRRPART, 0

Corresponding kernel trace:

[ 720.228114] INFO: task systemd-udevd:10134 blocked for more than 120 seconds.
[ 720.230656] Not tainted 3.16.0-28-generic #38-Ubuntu
[ 720.232666] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 720.236283] systemd-udevd D ffff88007fc13840 0 10134 1 0x00000007
[ 720.236293] ffff88007b76fcd0 0000000000000086 ffff880037641e90 0000000000013840
[ 720.236298] ffff88007b76ffd8 0000000000013840 ffff880037641e90 ffff880037641e90
[ 720.236302] ffff8800371c5068 ffff8800371c5080 ffff88007be723c0 0000000000000000
[ 720.236306] Call Trace:
[ 720.236320] [<ffffffff81785049>] schedule+0x29/0x70
[ 720.236326] [<ffffffff817888b5>] rwsem_down_read_failed+0xf5/0x190
[ 720.236334] [<ffffffff811fc743>] ? find_inode+0xa3/0xb0
[ 720.236341] [<ffffffff813a9284>] call_rwsem_down_read_failed+0x14/0x30
[ 720.236346] [<ffffffff817881e0>] ? down_read+0x20/0x30
[ 720.236351] [<ffffffff811e3db7>] get_super+0x97/0xe0
[ 720.236357] [<ffffffff812197a4>] fsync_bdev+0x14/0x60
[ 720.236363] [<ffffffff8137b604>] invalidate_partition+0x24/0x50
[ 720.236369] [<ffffffff8137e3cb>] drop_partitions.isra.12+0x1b/0x40
[ 720.236373] [<ffffffff8137e88e>] rescan_partitions+0x3e/0x2a0
[ 720.236377] [<ffffffff8137a4dc>] blkdev_ioctl+0x71c/0x7c0
[ 720.236382] [<ffffffff81218e3d>] block_ioctl+0x3d/0x50
[ 720.236386] [<ffffffff811f5138>] do_vfs_ioctl+0x2c8/0x4a0
[ 720.236393] [<ffffffff81082f30>] ? ptrace_notify+0x60/0x110
[ 720.236397] [<ffffffff811f5391>] SyS_ioctl+0x81/0xa0
[ 720.236402] [<ffffffff8178a0ff>] tracesys+0xe1/0xe6