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Subject: Re: 2.6.31-rc3 cpufreq bug (null pointer dereference)
From: "Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Lermytte Christophe <Christophe.Lermytte@thomson.net>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       "cpufreq@vger.kernel.org" <cpufreq@vger.kernel.org>,
       Dave Jones <davej@redhat.com>
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Date: Tue, 28 Jul 2009 16:41:51 -0700
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On Tue, 2009-07-28 at 16:22 -0700, Andrew Morton wrote:
> On Mon, 27 Jul 2009 10:20:02 +0200
> "Lermytte Christophe" <Christophe.Lermytte@thomson.net> wrote:
> 
> > > From: Andrew Morton [mailto:akpm@linux-foundation.org]
> > > Sent: Sat 7/25/2009 9:53 AM
> > > 
> > > (cc cpufreq@vger.kernel.org)
> > >
> > > > On Tue, 21 Jul 2009 22:07:49 +0200 Christophe Lermytte <christophe.lermytte@thomson.net> wrote:
> > > 
> > > > Today, I tried to change the governor to conservative using the GNOME
> > > > cpufreqd applet. The applet subsequently hangs and the following appears
> > > > in the kernel traces: 
> > > > 
> > > > [ 3067.249054] BUG: unable to handle kernel NULL pointer dereference at
> > > > 00000020
> > > > [ 3067.249062] IP: [<c1302d23>] dbs_cpufreq_notifier+0x17/0x2b
> > > > [ 3067.249074] *pde = 00000000 
> > > > [ 3067.249078] Oops: 0000 [#1] SMP 
> > > > [ 3067.249083] last sysfs
> > > > file: /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
> > > > [ 3067.249087] Modules linked in: cdc_wdm cdc_acm
> > > > [ 3067.249093] 
> > > > [ 3067.249098] Pid: 1346, comm: kondemand/1 Not tainted (2.6.31-rc3 #2)
> > > > Latitude D630                   
> > > > [ 3067.249103] EIP: 0060:[<c1302d23>] EFLAGS: 00010286 CPU: 1
> > > > [ 3067.249108] EIP is at dbs_cpufreq_notifier+0x17/0x2b
> > > > [ 3067.249112] EAX: 00000000 EBX: f6959f00 ECX: 00000000 EDX: c278c300
> > > > [ 3067.249116] ESI: fffffffe EDI: 00000000 EBP: 00000000 ESP: f6959e78
> > > > [ 3067.249120]  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
> > > > [ 3067.249124] Process kondemand/1 (pid: 1346, ti=f6958000 task=f69020a0
> > > > task.ti=f6958000)
> > > > [ 3067.249128] Stack:
> > > > [ 3067.249130]  c161ba54 c1427067 f6959f00 00000000 c176d000 f6959f00
> > > > c176cfe4 00000000
> > > > [ 3067.249139] <0> c103a57e ffffffff 00000000 00000000 00000001 f64de590
> > > > f8416980 f6959f00
> > > > [ 3067.249149] <0> c103a59c ffffffff 00000000 c100f8b0 f6bcf240 ffffffea
> > > > 00000003 00000000
> > > > [ 3067.249160] Call Trace:
> > > > [ 3067.249166]  [<c1427067>] ? notifier_call_chain+0x2a/0x47
> > > > [ 3067.249175]  [<c103a57e>] ? __srcu_notifier_call_chain+0x32/0x47
> > > > [ 3067.249181]  [<c103a59c>] ? srcu_notifier_call_chain+0x9/0xc
> > > > [ 3067.249193]  [<c100f8b0>] ? acpi_cpufreq_target+0x1a7/0x2b1
> > > > [ 3067.249197]  [<c100f5ba>] ? get_measured_perf+0x1f/0x140
> > > > [ 3067.249201]  [<c103db6d>] ? getnstimeofday+0x4d/0xd0
> > > > [ 3067.249205]  [<c1300802>] ? cpufreq_register_driver+0xb1/0x15b
> > > > [ 3067.249209]  [<c100f709>] ? acpi_cpufreq_target+0x0/0x2b1
> > > > [ 3067.249213]  [<c13008f5>] ? __cpufreq_driver_target+0x49/0x55
> > > > [ 3067.249217]  [<c13029a0>] ? do_dbs_timer+0x262/0x2b8
> > > > [ 3067.249222]  [<c1033b50>] ? worker_thread+0x137/0x1b3
> > > > [ 3067.249226]  [<c130273e>] ? do_dbs_timer+0x0/0x2b8
> > > > [ 3067.249229]  [<c1037042>] ? autoremove_wake_function+0x0/0x2d
> > > > [ 3067.249233]  [<c1033a19>] ? worker_thread+0x0/0x1b3
> > > > [ 3067.249236]  [<c1036da6>] ? kthread+0x69/0x6e
> > > > [ 3067.249240]  [<c1036d3d>] ? kthread+0x0/0x6e
> > > > [ 3067.249244]  [<c100334f>] ? kernel_thread_helper+0x7/0x10
> > > > [ 3067.249246] Code: c7 44 24 18 ea ff ff ff 8b 44 24 18 83 c4 20 5b 5e
> > > > 5f 5d c3 53 8b 01 ba 00 83 6c c1 89 cb 03 14 85 6c b9 66 c1 8b 4a 18 8b
> > > > 42 60 <3b> 41 20 77 05 3b 41 1c 73 06 8b 43 08 89 42 60 31 c0 5b c3 53 
> > > > [ 3067.249282] EIP: [<c1302d23>] dbs_cpufreq_notifier+0x17/0x2b SS:ESP
> > > > 0068:f6959e78
> > > > [ 3067.249287] CR2: 0000000000000020
> > > > [ 3067.249296] ---[ end trace 5cb870966b6883a7 ]---
> > > > 
> > > I assume that this is repeatable and that 2.6.30 was OK?
> > > 
> > > Thanks.
> > 
> > Yes, I do not encounter it on 2.6.30.2 and I can reproduce it continuously on 2.6.31-rc3 and 2.6.31-rc4.
> > 
> > Regards.
> 
> Thanks.  So we have a repeatable kernel-crashing post-2.6.30 regression.
> 
> this_dbs_info->cur_policy is NULL in dbs_cpufreq_notifier().


Looks like a bug in cpufreq_conservative.c
dbs_cpufreq_notifier() is a frequency change notifier function that
operates on policy structure without holding any lock. So, if policy
structure goes away for any reason, this is going to fail. Looking at
the code, there is nothing preventing this code from crashing in .30 as
well.

I am not entirely sure why we need this notifier in first place. Below
commit added it. But, we do not use any such notifier on
cpufreq_ondemand.c and things work fine there during suspend-resume.
If the policy limit changes, then governor notifier CPUFREQ_GOV_LIMITS
will be called anyway and I think this whole notifier is redundant as it
is again checking for the policy limits.

Dave: Do you remember about below change to conservative that added this
notifier thing?

Thanks,
Venki

commit a8d7c3bc2396aff14f9e920677072cb55b016040
Author: Elias Oltmanns <eo@nebensachen.de>
Date:   Mon Oct 22 09:50:13 2007 +0200

    [CPUFREQ] Make cpufreq_conservative handle out-of-sync events
properly
    
    Make cpufreq_conservative handle out-of-sync events properly
    
    Currently, the cpufreq_conservative governor doesn't get notified
when the
    actual frequency the cpu is running at differs from what cpufreq
thought it
    was. As a result the cpu may stay at the maximum frequency after a
s2ram /
    resume cycle even though the system is idle.
    
    Signed-off-by: Elias Oltmanns <eo@nebensachen.de>
    Signed-off-by: Dave Jones <davej@redhat.com>



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