mirror of
https://github.com/systemd/systemd.git
synced 2024-11-05 06:52:22 +03:00
b41a3f66c9
Add custom Jekyll theme, logo, webfont and .gitignore FIXME: the markdown files have some H1 headers which need to be replaced with H2
70 lines
3.7 KiB
Markdown
70 lines
3.7 KiB
Markdown
---
|
|
title: Locking Block Device Access
|
|
category: Interfaces
|
|
layout: default
|
|
---
|
|
|
|
# Locking Block Device Access
|
|
|
|
*TL;DR: Use BSD file locks
|
|
[(`flock(2)`)](http://man7.org/linux/man-pages/man2/flock.2.html) on block
|
|
device nodes to synchronize access for partitioning and file system formatting
|
|
tools.*
|
|
|
|
`systemd-udevd` probes all block devices showing up for file system superblock
|
|
and partition table information (utilizing `libblkid`). If another program
|
|
concurrently modifies a superblock or partition table this probing might be
|
|
affected, which is bad in itself, but also might in turn result in undesired
|
|
effects in programs subscribing to `udev` events.
|
|
|
|
Applications manipulating a block device can temporarily stop `systemd-udevd`
|
|
from processing rules on it — and thus bar it from probing the device — by
|
|
taking a BSD file lock on the block device node. Specifically, whenever
|
|
`systemd-udevd` starts processing a block device it takes a `LOCK_SH|LOCK_NB`
|
|
lock using [`flock(2)`](http://man7.org/linux/man-pages/man2/flock.2.html) on
|
|
the main block device (i.e. never on any partition block device, but on the
|
|
device the partition belongs to). If this lock cannot be taken (i.e. `flock()`
|
|
returns `EBUSY`), it refrains from processing the device. If it manages to take
|
|
the lock it is kept for the entire time the device is processed.
|
|
|
|
Note that `systemd-udevd` also watches all block device nodes it manages for
|
|
`inotify()` `IN_CLOSE` events: whenever such an event is seen, this is used as
|
|
trigger to re-run the rule-set for the device.
|
|
|
|
These two concepts allow tools such as disk partitioners or file system
|
|
formatting tools to safely and easily take exclusive ownership of a block
|
|
device while operating: before starting work on the block device, they should
|
|
take an `LOCK_EX` lock on it. This has two effects: first of all, in case
|
|
`systemd-udevd` is still processing the device the tool will wait for it to
|
|
finish. Second, after the lock is taken, it can be sure that
|
|
`systemd-udevd` will refrain from processing the block device, and thus all
|
|
other client applications subscribed to it won't get device notifications from
|
|
potentially half-written data either. After the operation is complete the
|
|
partitioner/formatter can simply close the device node. This has two effects:
|
|
it implicitly releases the lock, so that `systemd-udevd` can process events on
|
|
the device node again. Secondly, it results an `IN_CLOSE` event, which causes
|
|
`systemd-udevd` to immediately re-process the device — seeing all changes the
|
|
tool made — and notify subscribed clients about it.
|
|
|
|
Besides synchronizing block device access between `systemd-udevd` and such
|
|
tools this scheme may also be used to synchronize access between those tools
|
|
themselves. However, do note that `flock()` locks are advisory only. This means
|
|
if one tool honours this scheme and another tool does not, they will of course
|
|
not be synchronized properly, and might interfere with each other's work.
|
|
|
|
Note that the file locks follow the usual access semantics of BSD locks: since
|
|
`systemd-udevd` never writes to such block devices it only takes a `LOCK_SH`
|
|
*shared* lock. A program intending to make changes to the block device should
|
|
take a `LOCK_EX` *exclusive* lock instead. For further details, see the
|
|
`flock(2)` man page.
|
|
|
|
And please keep in mind: BSD file locks (`flock()`) and POSIX file locks
|
|
(`lockf()`, `F_SETLK`, …) are different concepts, and in their effect
|
|
orthogonal. The scheme discussed above uses the former and not the latter,
|
|
because these types of locks more closely match the required semantics.
|
|
|
|
Summarizing: it is recommended to take `LOCK_EX` BSD file locks when
|
|
manipulating block devices in all tools that change file system block devices
|
|
(`mkfs`, `fsck`, …) or partition tables (`fdisk`, `parted`, …), right after
|
|
opening the node.
|