Re: [PATCH] reboot: recognize and ignore -f/--force

[Sergey Bugaev]

On Tue, Oct 19, 2021 at 3:18 PM Samuel Thibault <samuel.thibault@gnu.org> wrote:
> IIRC the kernel does unmount filesystems and flushes caches before
> actually rebooting.

These two comments provide some more details:

"reboot doesn't sync: do that yourself before calling this." [0]
"The kernel will automatically flush ATA disks and suchlike on
reboot(), but the file systems need to be synced explicitly in
advance." [1]

[0]: 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/reboot.c#n309
[1]: 
https://github.com/systemd/systemd/blob/1eb3ef78b4df28a9e9f464714208f2682f957e36/src/systemctl/systemctl-util.c#L900-L901

To me, this reads like device-layer write caches will be flushed, but
any fs-layer caches won't, nor will filesystems be actually unmounted
or synced.

The point of 'reboot --force --force' is to be able to
programmatically reboot the system *somehow* if PID 1 is in a broken
state, assuming your shell and the kernel still work well enough to
run a command. To run any command on the Hurd, you'd need the root
filesystem and the proc and exec servers (and maybe the auth server)
to still work at least somewhat, plus of course the Mach. But maybe
something else is broken, maybe /hurd/startup has crashed, maybe
/servers/startup does not resolve for whatever reason (doesn't exist,
broken permissions, ext2fs bugs). That's when I'd expect 'reboot-hurd
--force' to be of use.

Sergey