Re: [linux-lvm] [PATCH 1/1] pvscan: wait for udevd

[Martin Wilck <mwilck@suse.com>]

On Wed, 2021-02-17 at 13:03 +0100, Christian Hesse wrote:
> 
> Let's keep this in mind. Now let's have a look at udevd startup: It
> signals
> being ready by calling sd_notifyf(), but it loads rules and applies
> permissions before doing so [0].
> Even before we have some code about handling events and monitoring
> stuff.

It loads the rules, but events will only be processed after entering
sd_event_loop(), which happens after the sd_notify() call.

Anyway, booting the system with "udev.log-priority=debug" might provide
further insight. Oleksandr, could you try that (without the After=
directive)?

> So I guess pvscan is started in initialization phase before udevd
> signals
> being ready. And obviously there is any kind of race condition.

Right. Some uevent might arrive between the creation of the monitor
socket in monitor_new() and entering the event loop. Such event would
be handled immediately, and possibly before systemd receives the
sd_notify message, so a race condition looks possible.

> 
> With the ordering "After=" in `lvm2-pvscan@.service` the service
> start is
> queued at initialization phase, but actual start and pvscan execution
> is
> delayed until udevd signaled being ready.
> 
> > But in general, I think this needs deeper analysis. Looking at
> > https://bugs.archlinux.org/task/69611, the workaround appears to
> > have
> > been found simply by drawing an analogy to a previous similar case.
> > I'd like to understand what happened on the arch system when the
> > error
> > occured, and why this simple ordering directive avoided it.
> 
> As said I can not reproduce it myself... Oleksandr, can you give more
> details?
> Possibly everything from journal regarding systemd-udevd.service (and
> systemd-udevd.socket) and lvm2-pvscan@*.service could help.
> 
> > 1. How had the offending pvscan process been started? I'd expect
> > that
> > "pvscan" (unlike "lvm monitor" in our case) was started by an udev
> > rule. If udevd hadn't started yet, how would that udev rule have be
> > executed? OTOH, if pvscan had not been started by udev but by
> > another
> > systemd service, than *that* service would probably need to get the
> > After=systemd-udevd.service directive.
> 
> To my understanding it was started from udevd by a rule in
> `69-dm-lvm-metad.rules`.
> 
> (BTW, renaming that rule file may make sense now that lvm2-metad is
> gone...)
> 
> > 2. Even without the "After=" directive, I'd assume that pvscan
> > wasn't
> > started "before" systemd-udevd, but rather "simultaneously" (i.e.
> > in
> > the same systemd transaction). Thus systemd-udevd should have
> > started
> > up while pvscan was running, and pvscan should have noticed that
> > udevd
> > eventually became available. Why did pvscan time out? What was it
> > waiting for? We know that lvm checks for the existence of
> > "/run/udev/control", but that should have become avaiable after
> > some
> > fractions of a second of waiting.
> 
> I do not think there is anything starting pvscan before udevd.

I agree. The race described above looks at least possible.
I would go one step further and say that *every* systemd service that
might be started from an udev rule should have an "After=systemd-
udevd.service".

Martin


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