--- 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. # Example of Locking The Whole Disk The following is an example to leverage `libsystemd` infrastructure to get the whole disk of a block device and take a BSD lock on it. ## Compile and Execute **Note that this example requires `libsystemd` version 251 or newer.** Place the code in a source file, e.g. `take_BSD_lock.c` and run the following commands: ``` $ gcc -o take_BSD_lock -lsystemd take_BSD_lock.c $ ./take_BSD_lock /dev/sda1 Successfully took a BSD lock: /dev/sda $ flock -x /dev/sda ./take_BSD_lock /dev/sda1 Failed to take a BSD lock on /dev/sda: Resource temporarily unavailable ``` ## Code ```c /* SPDX-License-Identifier: MIT-0 */ #include #include #include #include #include #include static inline void closep(int *fd) { if (*fd >= 0) close(*fd); } /** * lock_whole_disk_from_devname * @devname: devname of a block device, e.g., /dev/sda or /dev/sda1 * @open_flags: the flags to open the device, e.g., O_RDONLY|O_CLOEXEC|O_NONBLOCK|O_NOCTTY * @flock_operation: the operation to call flock, e.g., LOCK_EX|LOCK_NB * * given the devname of a block device, take a BSD lock of the whole disk * * Returns: negative errno value on error, or non-negative fd if the lock was taken successfully. **/ int lock_whole_disk_from_devname(const char *devname, int open_flags, int flock_operation) { __attribute__((cleanup(sd_device_unrefp))) sd_device *dev = NULL; sd_device *whole_dev; const char *whole_disk_devname, *subsystem, *devtype; int r; // create a sd_device instance from devname r = sd_device_new_from_devname(&dev, devname); if (r < 0) { errno = -r; fprintf(stderr, "Failed to create sd_device: %m\n"); return r; } // if the subsystem of dev is block, but its devtype is not disk, find its parent r = sd_device_get_subsystem(dev, &subsystem); if (r < 0) { errno = -r; fprintf(stderr, "Failed to get the subsystem: %m\n"); return r; } if (strcmp(subsystem, "block") != 0) { fprintf(stderr, "%s is not a block device, refusing.\n", devname); return -EINVAL; } r = sd_device_get_devtype(dev, &devtype); if (r < 0) { errno = -r; fprintf(stderr, "Failed to get the devtype: %m\n"); return r; } if (strcmp(devtype, "disk") == 0) whole_dev = dev; else { r = sd_device_get_parent_with_subsystem_devtype(dev, "block", "disk", &whole_dev); if (r < 0) { errno = -r; fprintf(stderr, "Failed to get the parent device: %m\n"); return r; } } // open the whole disk device node __attribute__((cleanup(closep))) int fd = sd_device_open(whole_dev, open_flags); if (fd < 0) { errno = -fd; fprintf(stderr, "Failed to open the device: %m\n"); return fd; } // get the whole disk devname r = sd_device_get_devname(whole_dev, &whole_disk_devname); if (r < 0) { errno = -r; fprintf(stderr, "Failed to get the whole disk name: %m\n"); return r; } // take a BSD lock of the whole disk device node if (flock(fd, flock_operation) < 0) { r = -errno; fprintf(stderr, "Failed to take a BSD lock on %s: %m\n", whole_disk_devname); return r; } printf("Successfully took a BSD lock: %s\n", whole_disk_devname); // take the fd to avoid automatic cleanup int ret_fd = fd; fd = -EBADF; return ret_fd; } int main(int argc, char **argv) { if (argc != 2) { fprintf(stderr, "Invalid number of parameters.\n"); return EXIT_FAILURE; } // try to take an exclusive and nonblocking BSD lock __attribute__((cleanup(closep))) int fd = lock_whole_disk_from_devname( argv[1], O_RDONLY|O_CLOEXEC|O_NONBLOCK|O_NOCTTY, LOCK_EX|LOCK_NB); if (fd < 0) return EXIT_FAILURE; /** * The device is now locked until the return below. * Now you can safely manipulate the block device. **/ return EXIT_SUCCESS; } ```