mirror of
https://github.com/cloud-hypervisor/cloud-hypervisor.git
synced 2026-08-05 02:19:16 +00:00
Philipp Schuster
05968f5c2c
# What This commit introduces file-based advisory locking for the files backing up the block devices by using the fcntl() syscall with OFD locks. The per-open-file-descriptor (OFD) locks are more robust than traditional POSIX locks (F_SETLK) as they are not tied to process IDs and avoid common issues in multithreaded or multi-fd scenarios [1]. Therefore, we don't use `std::fs::File::try_lock()`, which is backed by F_SETLKW. The locking mechanism is aware of the `readonly` property and allows `n` readers or `1` writer (exclusive mode). As the locks are advisory, multiple cloud-hypervisor processes can prevent themselves from writing to the same file. However, this is not a system-wide file-system level locking mechanism preventing to open() a file. The introduced new locking mechanism does not cover vhost-user devices. # Why To prevent misconfiguration and improve safety, it is good practice to protect disk image files with a locking mechanism. Experience and common best practices suggest that advisory locks are preferable over mandatory locks due to better compatibility and fewer pitfalls (in fs space). The introduced functionality is aligned with the approach taken by QEMU [0], and is also recommended in [1]. # Implementation Details We need to ensure that not only normal operation keeps working but also state save/resume and live-migration. Especially for live migration, it is crucial that the sender VMM releases the locks when the VM stops so the receiver VMM can acquire them right after that. Therefore, the locking and releasing happen directly on the block device struct. The device manager knows all block devices and can forward requests to these types. Last but not least, this commit uses on explicit lock acquiring but implicit lock releasing (FD close). It only explicitly releases the locks where this integrates more smoothly into the existing code. # Testing I tested - normal operation - state save/resume, - device hot plugging, - and live-migration with read/shared and write/exclusive locks. One can use the `fcntl-tool` to test if locks are actually acquired or released [2]. # Links [0] https://github.com/qemu/qemu/blob/825b96dbcee23d134b691fc75618b59c5f53da32/util/osdep.c#L266 [1] https://apenwarr.ca/log/20101213 [2] https://crates.io/crates/fcntl-tool Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de> On-behalf-of: SAP philipp.schuster@sap.com