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diff --git a/docs/BLOCK_DEVICE_LOCKING.md b/docs/BLOCK_DEVICE_LOCKING.md new file mode 100644 index 0000000..61beff0 --- /dev/null +++ b/docs/BLOCK_DEVICE_LOCKING.md @@ -0,0 +1,100 @@ +--- +title: Locking Block Device Access +category: Interfaces +layout: default +SPDX-License-Identifier: LGPL-2.1-or-later +--- + +# Locking Block Device Access + +*TL;DR: Use BSD file locks +[(`flock(2)`)](https://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)`](https://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 `EAGAIN`), 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_WRITE` 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_WRITE` event, which +causes `systemd-udevd` to immediately re-process the device — seeing all +changes the tool made — and notify subscribed clients about it. + +Ideally, `systemd-udevd` would explicitly watch block devices for `LOCK_EX` +locks being released. Such monitoring is not supported on Linux however, which +is why it watches for `IN_CLOSE_WRITE` instead, i.e. for `close()` calls to +writable file descriptors referring to the block device. In almost all cases, +the difference between these two events does not matter much, as any locks +taken are implicitly released by `close()`. However, it should be noted that if +an application unlocks a device after completing its work without closing it, +i.e. while keeping the file descriptor open for further, longer time, then +`systemd-udevd` will not notice this and not retrigger and thus reprobe the +device. + +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. + +If multiple devices are to be locked at the same time (for example in order to +format a RAID file system), the devices should be locked in the order of the +the device nodes' major numbers (primary ordering key, ascending) and minor +numbers (secondary ordering key, ditto), in order to avoid ABBA locking issues +between subsystems. + +Note that the locks should only be taken while the device is repartitioned, +file systems formatted or `dd`'ed in, and similar cases that +apply/remove/change superblocks/partition information. It should not be held +during normal operation, i.e. while file systems on it are mounted for +application use. + +The [`udevadm +lock`](https://www.freedesktop.org/software/systemd/man/udevadm.html) command +is provided to lock block devices following this scheme from the command line, +for the use in scripts and similar. (Note though that it's typically preferable +to use native support for block device locking in tools where that's +available.) + +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. |