[systemd-devel] more verbose debug info than systemd.log_level=debug?

[Michael Chapman]

On Mon, 10 Apr 2017, Lennart Poettering wrote:
> On Sun, 09.04.17 10:11, Michael Chapman (mike at very.puzzling.org) wrote:
>
>> Don't forget, they've provided an interface for software to use if it needs
>> more than the guarantees provided by sync. Informally speaking, the FIFREEZE
>> ioctl is intended to place a filesystem into a "fully consistent" state, not
>> just a "fully recoverable" state. (Formally it's all a bit hazy: POSIX
>> really doesn't guarantee anything with sync.)
>
> FIFREEZE does considerably more than what you suggest: it also pauses
> all further changes until FITHAW is called. And that's semantics we
> really cannot have.

If systemd is just about to call reboot(2), why does it matter?

I do think we should attempt to remount readonly before doing the 
FIFREEZE. I thought systemd did that, but it appears that it does not. A 
readonly remount will do what we want so long as no remaining processes 
have any files opened for writing on the filesystem. The FIFREEZE would 
only be necessary when the remount fails.

Remember, all of this is because there *is* software that does the wrong 
thing, and it *is* possible for software to hang and be unkillable. It 
would be good for systemd to do the right thing even in the presence of 
that kind of software.