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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 15 Nov 2010 17:18:50 -0800
Message-ID: <AANLkTik53N7HMyXMOEzh2Qr_4gg9eXSmXiT6TQ9gFC+h@mail.gmail.com>
Subject: Re: [RFC/RFT PATCH v3] sched: automated per tty task groups
To: Mike Galbraith <efault@gmx.de>
Cc: Valdis.Kletnieks@vt.edu, Oleg Nesterov <oleg@redhat.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>,
        Markus Trippelsdorf <markus@trippelsdorf.de>,
        Daniel Lezcano <daniel.lezcano@free.fr>,
        Paul Menage <menage@google.com>
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Hmm. Just found a bug. I'm not sure if it's the autogroup patches
themselves, or whether it's just the cgroup code that the autogroup
patch enables for me.

When I do

   echo t > /proc/sysrq-trigger

(or "w") I get a NULL pointer dereference (offset 0x38 - decimal 56)
in "cgroup_path+0x7", with a call trace of sched_debug_show,
show_state_filter, sysrq_handle_showstate_blocked. I don't have the
whole oops, because the machine is really dead at that point
(presumably died holding the runqueue lock or some other critical
resource), but if required I could take a photo of it. However, I bet
it is repeatable, so I doubt you need it.

Anyway, that "cgroup_path+0x7" is the very first memory dereference:

    movq    56(%rdi), %rsi  # cgrp_5(D)->dentry, _________p1

so sched_debug_show() is apparently calling cgroup_path() with a NULL
cgroup. I think it's "print_task()" that is to blame, it does

     cgroup_path(task_group(p)->css.cgroup, ..

without checking whether there _is_ any css.cgroup.

Peter, that looks like your code (commit d19ca30874f2)

Guys?

                      Linus
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