linux-kernel - Re: [PATCH cgroup/for-3.16] cgroup: introduce CSS_NO_REF and skip refcnting on normal root csses

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Date:	Fri, 16 May 2014 10:59:37 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Tejun Heo <tj@...nel.org>
CC:	lizefan@...wei.com, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	ARM kernel mailing list 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH cgroup/for-3.16] cgroup: introduce CSS_NO_REF and skip
 refcnting on normal root csses

On 05/16/2014 09:43 AM, Tejun Heo wrote:
> From 49035d695d8f1c4bbe4e37480e5d06812818947c Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj@...nel.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@...nel.org>
> Reported-by: Stephen Warren <swarren@...dotorg.org>
> 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.
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