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

[Lennart Poettering]

On Mon, 10.04.17 16:14, Michael Chapman (mike at very.puzzling.org) wrote:

> On Mon, 10 Apr 2017, Chris Murphy wrote:
> > On Sun, Apr 9, 2017 at 5:17 AM, Lennart Poettering
> > <lennart at poettering.net> wrote:
> > 
> > > That said, are you sure FIFREEZE is really what we want there? it
> > > appears to also pause any further writes to disk (until FITHAW is
> > > called).
> > 
> > > So, I am still puzzled why the file system people think that "sync()"
> > > isn't supposed to actually sync things to disk...
> > 
> > https://www.spinics.net/lists/linux-xfs/msg05113.html
> 
> Ah good, Dave actually suggests using a freeze there. A freeze without a
> corresponding thaw should be OK if it's definitely after all processes have
> been killed, since we're just about to reboot anyway. (Obviously we'd want
> to avoid the whole lot when running in a container or when doing
> kexec.)

No, there is no such guarantee. We support initrds that run userspace
stuff from the initrd at boot, that stays around in the background is
only killed after we transition back into the initrd. And we really
don't control what they do, they can do anything they like, access any
file they want at any time. We added this primarily to support storage
services backing the root file system (think iscsid, nbd, ...), but it
actually can be anything that hsa the "feel" of a kernel component in
being around since the time before systemd initialiazes until after
the time it shut down again, but is actually implemented in userspace.

In fact, this is precisely what plymouth is making use of: by marking
a process with argv[0][0] = '@' we permit any privileged process to be
excluded from the final killing spree, so that it will survive until
the initrd shutdown transition.

So no, "freeze" is not an option. That sounds like a recipe to make
shutdown hang. We need a sync() that actually does what is documented
and sync the file system properly.

Lennart

-- 
Lennart Poettering, Red Hat