IME, flock works fine across SIGKILLs and forced reboots. Are you trying to use flock on a filesystem that runs across multiple kernels (i.e., networked fs), by

IME, flock works fine across SIGKILLs and forced reboots. Are you trying to use flock on a filesystem that runs across multiple kernels (i.e., networked fs), by any chance?