linux-kernel - Re: cgroup: BUG: unable to handle kernel NULL pointer dereference

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Date:	Sat, 2 Jan 2016 12:24:16 -0600
From:	"Serge E. Hallyn" <serge.hallyn@...ntu.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jeremiah Mahler <jmmahler@...il.com>, cgroups@...r.kernel.org,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	linux-kernel@...r.kernel.org,
	Dan Williams <dan.j.williams@...el.com>
Subject: Re: cgroup: BUG: unable to handle kernel NULL pointer dereference

On Sat, Jan 02, 2016 at 06:54:37AM -0500, Tejun Heo wrote:
> On Fri, Jan 01, 2016 at 03:40:28PM -0800, Jeremiah Mahler wrote:
> > all,
> > 
> > When running the latest linux-next (20151231) two of my machines
> > hang early in the boot sequence.  The initial message is for a
> > NULL pointer dereference.
> > 
> >   BUG: unable to handle kernel NULL pointer dereference at 0000000000000030
> > 
> > And the RIP line refers to cgroup_path.
> > 
> >   RIP [<ffffffff810eb178>] cgroup_path+0x30/0x80
> > 
> > Attached are pictures of the back trace.
> > 
> > Let me know if I can do anything else to help.  I will investigate the
> > problem further if I get a chance.
> 
> This is most likely from the recent changes from cgroup ns support.
> Reverted the patchset for now as it also introduced a bisectability
> issue.  Serge, can you please look into this one?

Tried to reproduce with setting CONFIG_CFQ_GROUP_IOSCHED=y, but did not
succeed.  Could you send me the .config?  Also, if someone could send
the objdump -d output that might help.  Though really, it seems clear
that current->nsproxy must be NULL.  Hm, that's right -  we used to have
that issue in pidns (or was it netns) during process exit.  I don't know
that I'll get time this afternoon, but I'll look into it asap.

thanks.
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