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From:   Aubrey Li <aubrey.intel@gmail.com>
Date:   Thu, 14 Mar 2019 13:30:03 +0800
Message-ID: <CAERHkrvFe-s=_SD2WO0yuYiQ3JxY22=pSrM4UE-BWpbPD8iiCA@mail.gmail.com>
Subject: Re: [RFC][PATCH 00/16] sched: Core scheduling
To:     Tim Chen <tim.c.chen@linux.intel.com>
Cc:     Subhra Mazumdar <subhra.mazumdar@oracle.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul Turner <pjt@google.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Fr?d?ric Weisbecker" <fweisbec@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Greg Kerr <kerrnel@google.com>
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On Thu, Mar 14, 2019 at 8:35 AM Tim Chen <tim.c.chen@linux.intel.com> wrote:
> >>
> >> One more NULL pointer dereference:
> >>
> >> Mar 12 02:24:46 aubrey-ivb kernel: [  201.916741] core sched enabled
> >> [  201.950203] BUG: unable to handle kernel NULL pointer dereference
> >> at 0000000000000008
> >> [  201.950254] ------------[ cut here ]------------
> >> [  201.959045] #PF error: [normal kernel read fault]
> >> [  201.964272] !se->on_rq
> >> [  201.964287] WARNING: CPU: 22 PID: 2965 at kernel/sched/fair.c:6849
> >> set_next_buddy+0x52/0x70
> >
> Shouldn't the for_each_sched_entity(se) skip the code block for !se case
> have avoided null pointer access of se?
>
> Since
> #define for_each_sched_entity(se) \
>                 for (; se; se = se->parent)
>
> Scratching my head a bit here on how your changes would have made
> a difference.

This NULL pointer dereference is not replicable, which makes me thought the
change works...

>
> In your original log, I wonder if the !se->on_rq warning on CPU 22 is mixed with the actual OOPs?
> Saw also in your original log rb_insert_color.  Wonder if that
> was actually the source of the Oops?

No chance to figure this out, I only saw this once, lockup occurs more
frequently.

Thanks,
-Aubrey