Date: Wed, 13 Dec 2017 13:37:56 -0800
From: Tejun Heo <tj@kernel.org>
To: Laura Abbott <labbott@redhat.com>
Cc: Zefan Li <lizefan@huawei.com>, linux-kernel@vger.kernel.org,
        regressions@leemhuis.info
Subject: Re: Crash in cgroup_procs_show
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Hello, Laura.

On Wed, Dec 13, 2017 at 09:10:36AM -0800, Laura Abbott wrote:
> On 12/13/2017 07:29 AM, Tejun Heo wrote:
> >Hello, Laura.
> >
> >On Tue, Dec 12, 2017 at 04:38:33PM -0800, Laura Abbott wrote:
> >>Hi,
> >>
> >>Fedora got a bug report on 4.14.4 of a crash on
> >>reboot https://bugzilla.redhat.com/show_bug.cgi?id=1525279
> >>
> >>  BUG: unable to handle kernel NULL pointer dereference at 0000000000000254
> >>  IP: __task_pid_nr_ns+0xc7/0xf0
> >
> >Any chance you can map this back to the source line with addr2line?
> >
> >Thanks.
> >
> 
> annotated with decode_stacktrace.sh
> 
> 
> IP: __task_pid_nr_ns (/usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/kernel/pid.c:506 /usr/src/debug/kernel-4.14.fc26/linux-4.14.4-200.fc26.x86_64/kernel/pid.c:535)
 
Thank you very much for the trace.  I *think* I found the place where
the css task iterator points to a task without holding its ref
allowing the task to go away before dereferenced.  I'll update once I
learn more.

Thanks.

-- 
tejun