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

[Jessica Clarke]

On 19 Oct 2021, at 13:41, Sergey Bugaev <bugaevc@gmail.com> wrote:
> 
> 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.

[0] may be of interest.

Jess

[0]: https://bsdimp.blogspot.com/2020/07/when-unix-learned-to-reboot2.html