From: "Luis R. Rodriguez" <redacted>
Date: Mon, 28 Jul 2014 11:28:28 -0700

Tetsuo bisected and found that commit 786235ee "kthread: make
kthread_create() killable" modified kthread_create() to bail as
soon as SIGKILL is received. This is causing some issues with
some drivers and at times boot. Joseph then found that failures
occur as the systemd-udevd process sends SIGKILL to modprobe if
probe on a driver takes over 30 seconds. When this happens probe
will fail on any driver, its why booting on some system will fail
if the driver happens to be a storage related driver. Some folks
have suggested fixing this by modifying kthread_create() to not
leave upon SIGKILL [3], upon review Oleg rejected this change and
the discussion was punted out to systemd to see if the default
timeout could be increased from 30 seconds to 120. The opinion of
the systemd maintainers is that the driver's behavior should
be fixed [4]. Linus seems to agree [5], however more recently even
networking drivers have been reported to fail on probe since just
writing the firmware to a device and kicking it can take easy over
60 seconds [6]. Benjamim was able to trace the issues recently
reported on cxgb4 down to the same systemd-udevd 30 second timeout [6].

This is an alternative solution which enables drivers that are
known to take long to use deferred probe workqueue. This avoids
the 30 second timeout and lets us annotate drivers with long
init sequences.

As drivers determine a component is not yet available and needs
to defer probe you'll be notified this happen upon init for each
device but now with a message such as:

pci 0000:03:00.0: Driver cxgb4 requests probe deferral on init

You should see one of these per struct device probed.

It seems we're still discussing this.

I think I understand all of the underlying issues, and what I'll say
is that perhaps we should use what Greg KH requested but via a helper
that is easy to grep for.

I don't care if it's something like "module_long_probe_init()" and
"module_long_probe_exit()", but it just needs to be some properly
named interface which does the whole kthread or whatever bit.

Thanks.