linux-kernel - Re: tg_load_down NULL pointer dereference

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Message-ID: <20160620083417.GE30154@twins.programming.kicks-ass.net>
Date:	Mon, 20 Jun 2016 10:34:17 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	AMIT NAGAL <amit.nagal@...sung.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com,
	ajeet.y@...sung.com, pankaj.m@...sung.com
Subject: Re: tg_load_down NULL pointer dereference

On Mon, Jun 20, 2016 at 06:14:01AM +0000, AMIT NAGAL wrote:
> Hi 
> I am using  Linux kernel version 3.10.28 (ARM platform) .
> I am getting NULL pointer dereference in tg_load_down() .
> At the time of error  , tg->parent->cfs_rq  value is 0 and  tg->se value is 0x00000400 . ( refer to backtrace in 5) ).

Were you destroying cgroups at the time?

If so, there were some problems with cgroup teardown recently, see
patches:

  6fe1f348b3dd ("sched/cgroup: Fix cgroup entity load tracking tear-down")
  2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")

Which depend on:

  aa226ff4a1ce ("cgroup: make sure a parent css isn't offlined before its children")
  8bb5ef79bc0f ("cgroup: make sure a parent css isn't freed before its children")

I've no idea if any of that is relevant to your ancient kernel, let
alone applies, that's your problem for using ancient wares.

If you can reproduce with a current kernel (4.6+) then I might look more.