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So here's something we should always keep in mind: systemd-udevd actually does *two* things with BSD file locks on block devices: 1. While it probes a device it takes a LOCK_SH lock. Thus everyone else taking a LOCK_EX lock will temporarily block udev from probing devices, which is good when making changes to it. 2. Whenever a device is closed after write (detected via inotify), udevd will issue BLKRRPART (requesting the kernel to reread the partition table). It does this while holding a LOCK_EX lock on the block device. Thus anyone else taking LOCK_SH or LOCK_EX will temporarily block udevd from issuing that ioctl. And that's quite relevant, since the kernel will temporarily flush out all partitions while re-reading the partition table and then create them anew. Thus it is smart to take LOCK_SH when dissecting a block device to ensure that no BLKRRPART is issued in the background, until we mounted the devices.