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To:     Aubrey Li <aubrey.intel@gmail.com>,
        Subhra Mazumdar <subhra.mazumdar@oracle.com>
Cc:     Mel Gorman <mgorman@techsingularity.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Fr?d?ric Weisbecker <fweisbec@gmail.com>,
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        Greg Kerr <kerrnel@google.com>
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From:   Tim Chen <tim.c.chen@linux.intel.com>
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Subject: Re: [RFC][PATCH 00/16] sched: Core scheduling
Message-ID: <8098aac2-60f7-6fe9-2a3a-2fe2e1b49bde@linux.intel.com>
Date:   Wed, 13 Mar 2019 17:35:13 -0700
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>>
>> 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
> 
> A quick workaround below:
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1d0dac4fd94f..ef6acfe2cf7d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6834,7 +6834,7 @@ static void set_last_buddy(struct sched_entity *se)
>                 return;
> 
>         for_each_sched_entity(se) {
> -               if (SCHED_WARN_ON(!se->on_rq))
> +               if (SCHED_WARN_ON(!(se && se->on_rq))
>                         return;
>                 cfs_rq_of(se)->last = se;
>         }
> @@ -6846,7 +6846,7 @@ static void set_next_buddy(struct sched_entity *se)
>                 return;
> 
>         for_each_sched_entity(se) {
> -               if (SCHED_WARN_ON(!se->on_rq))
> +               if (SCHED_WARN_ON(!(se && se->on_rq))


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.

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?


[  202.078674] RIP: 0010:set_next_buddy+0x52/0x70
[  202.090135] Hardware name: Intel Corporation S2600CP/S2600CP, BIOS
SE5C600.86B.99.99.x058.082120120902 08/21/2012
[  202.090144] RIP: 0010:rb_insert_color+0x17/0x190
[  202.101623] Code: 48 85 ff 74 10 8b 47 40 85 c0 75 e2 80 3d 9e e5
6a 01 00 74 02 f3 c3 48 c7 c7 5c 05 2c 82 c6 05 8c e5 6a 01 01 e8 2e
bb fb ff <0f> 0b c3 83 bf 04 03 0e
[  202.113216] Code: f3 c3 31 c0 c3 0f 1f 40 00 66 2e 0f 1f 84 00 00
00 00 00 48 8b 17 48 85 d2 0f 84 4d 01 00 00 48 8b 02 a8 01 0f 85 6d
01 00 00 <48> 8b 48 08 49 89 c0 44
[  202.118263] RSP: 0018:ffffc9000a5cbbb0 EFLAGS: 00010086
[  202.129858] RSP: 0018:ffffc9000a463cc0 EFLAGS: 00010046
[  202.135102] RAX: 0000000000000000 RBX: ffff88980047e800 RCX: 0000000000000000
[  202.135105] RDX: ffff888be28caa40 RSI: 0000000000000001 RDI: ffffffff8110c3fa
[  202.156251] RAX: 0000000000000000 RBX: ffff888bfeb80000 RCX: ffff888bfeb80

Thanks.

Tim