c3e270f4ee
It turns out Jekyll (the engine behind GitHub Pages) requires that pages
include a "Front Matter" snippet of YAML at the top for proper rendering.
Omitting it will still render the pages, but including it opens up new
possibilities, such as using a {% for %} loop to generate index.md instead of
requiring a separate script.
I'm hoping this will also fix the issue with some of the pages (notably
CODE_OF_CONDUCT.html) not being available under systemd.io
Tested locally by rendering the website with Jekyll. Before this change, the
*.md files were kept unchanged (so not sure how that even works?!), after this
commit, proper *.html files were generated from it.
68 lines
3.7 KiB
Markdown
68 lines
3.7 KiB
Markdown
---
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title: Locking Block Device Access
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---
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# Locking Block Device Access
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*TL;DR: Use BSD file locks
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[(`flock(2)`)](http://man7.org/linux/man-pages/man2/flock.2.html) on block
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device nodes to synchronize access for partitioning and file system formatting
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tools.*
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`systemd-udevd` probes all block devices showing up for file system superblock
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and partition table information (utilizing `libblkid`). If another program
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concurrently modifies a superblock or partition table this probing might be
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affected, which is bad in itself, but also might in turn result in undesired
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effects in programs subscribing to `udev` events.
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Applications manipulating a block device can temporarily stop `systemd-udevd`
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from processing rules on it — and thus bar it from probing the device — by
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taking a BSD file lock on the block device node. Specifically, whenever
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`systemd-udevd` starts processing a block device it takes a `LOCK_SH|LOCK_NB`
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lock using [`flock(2)`](http://man7.org/linux/man-pages/man2/flock.2.html) on
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the main block device (i.e. never on any partition block device, but on the
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device the partition belongs to). If this lock cannot be taken (i.e. `flock()`
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returns `EBUSY`), it refrains from processing the device. If it manages to take
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the lock it is kept for the entire time the device is processed.
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Note that `systemd-udevd` also watches all block device nodes it manages for
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`inotify()` `IN_CLOSE` events: whenever such an event is seen, this is used as
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trigger to re-run the rule-set for the device.
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These two concepts allow tools such as disk partitioners or file system
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formatting tools to safely and easily take exclusive ownership of a block
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device while operating: before starting work on the block device, they should
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take an `LOCK_EX` lock on it. This has two effects: first of all, in case
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`systemd-udevd` is still processing the device the tool will wait for it to
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finish. Second, after the lock is taken, it can be sure that that
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`systemd-udevd` will refrain from processing the block device, and thus all
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other client applications subscribed to it won't get device notifications from
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potentially half-written data either. After the operation is complete the
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partitioner/formatter can simply close the device node. This has two effects:
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it implicitly releases the lock, so that `systemd-udevd` can process events on
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the device node again. Secondly, it results an `IN_CLOSE` event, which causes
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`systemd-udevd` to immediately re-process the device — seeing all changes the
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tool made — and notify subscribed clients about it.
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Besides synchronizing block device access between `systemd-udevd` and such
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tools this scheme may also be used to synchronize access between those tools
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themselves. However, do note that `flock()` locks are advisory only. This means
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if one tool honours this scheme and another tool does not, they will of course
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not be synchronized properly, and might interfere with each other's work.
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Note that the file locks follow the usual access semantics of BSD locks: since
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`systemd-udevd` never writes to such block devices it only takes a `LOCK_SH`
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*shared* lock. A program intending to make changes to the block device should
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take a `LOCK_EX` *exclusive* lock instead. For further details, see the
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`flock(2)` man page.
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And please keep in mind: BSD file locks (`flock()`) and POSIX file locks
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(`lockf()`, `F_SETLK`, …) are different concepts, and in their effect
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orthogonal. The scheme discussed above uses the former and not the latter,
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because these types of locks more closely match the required semantics.
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Summarizing: it is recommended to take `LOCK_EX` BSD file locks when
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manipulating block devices in all tools that change file system block devices
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(`mkfs`, `fsck`, …) or partition tables (`fdisk`, `parted`, …), right after
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opening the node.
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