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.
Lennart Poettering
41bc484906