Lennart Poettering 41bc484906 tree-wide: take BSD lock on loopback devices we dissect/mount/operate on ...

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.

2022-04-10 22:52:29 +09:00

..

dissect.c

tree-wide: take BSD lock on loopback devices we dissect/mount/operate on

2022-04-10 22:52:29 +09:00