On 05/16/2014 09:43 AM, Tejun Heo wrote:

From 49035d695d8f1c4bbe4e37480e5d06812818947c Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Fri, 16 May 2014 11:40:33 -0400

9395a4500404 ("cgroup: enable refcnting for root csses") enabled
reference counting for root csses (cgroup_subsys_states) so that
cgroup's self csses can be used to manage the lifetime of the
containing cgroups.

Unfortunately, this change was incorrect.  cpu controller uses
early_init and starts using css reference counts on its root css from
then on.  percpu_ref can't initialized during early init and its
initialization is deferred till cgroup_init() time.  This means that
cpu was using percpu_ref which wasn't properly initialized.  Due to
the way percpu variables are laid out on x86, this didn't blow up
immediately on x86 but ended up incrementing and decrementing the
percpu variable at offset zero, whatever it may be; however, on other
archs, this caused fault and early boot failure.

As cgroup self csses still need working refcounting, we can't revert
9395a4500404.  This patch adds CSS_NO_REF which explicitly inhibits
reference counting on the css and sets it on all normal (non-self)
csses.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Stephen Warren <redacted>
Fixes: 9395a4500404 ("cgroup: enable refcnting for root csses")
---
Patch applied to cgroup/for-3.16.

Unfortunately, this patch doesn't seem to solve the problem for me. I
still get a hang/crash very early during boot, before earlyprintk stars.

I tried this patch on top of both next-20150115, and next-20150516. For
both those kernels, reverting the original problematic patch does
resolve the problem.

I'll try to investigate more later today.