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Date: Thu, 24 May 2012 17:14:07 -0700
From: Andi Kleen <ak@linux.intel.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Dave Jones <davej@redhat.com>, Linux Kernel <linux-kernel@vger.kernel.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@linux.intel.com>
Subject: Re: tracing ring_buffer_resize oops.
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On Thu, May 24, 2012 at 07:40:16PM -0400, Steven Rostedt wrote:
> On Thu, 2012-05-24 at 13:22 -0400, Dave Jones wrote:
> 
> I found a clue!
> 
> 
> > [ 1013.243754] BUG: unable to handle kernel NULL pointer dereference at 0000000000000002
> > [ 1013.272665] IP: [<ffff880145cc0000>] 0xffff880145cbffff
> > [ 1013.285186] PGD 1401b2067 PUD 14324c067 PMD 0 
> > [ 1013.298832] Oops: 0010 [#1] PREEMPT SMP 
> > [ 1013.310600] CPU 2 
> > [ 1013.317904] Modules linked in: ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 xt_state nf_conntrack ip6table_filter ip6_tables crc32c_intel ghash_clmulni_intel microcode usb_debug serio_raw pcspkr iTCO_wdt i2c_i801 iTCO_vendor_support e1000e nfsd nfs_acl auth_rpcgss lockd sunrpc i915 video i2c_algo_bit drm_kms_helper drm i2c_core [last unloaded: scsi_wait_scan]
> > [ 1013.401848] 
> > [ 1013.407399] Pid: 112, comm: kworker/2:1 Not tainted 3.4.0+ #30 
> > [ 1013.437943] RIP: 8eb8:[<ffff88014630a000>]  [<ffff88014630a000>] 0xffff880146309fff
> 
> RIP is always near the GS segment. As GS points to the per_cpu area, we
> may somehow be getting our GS screwed up. I'm not sure why that would
> affect the RIP. Maybe stacks are not being processed properly somewhere?
> 
> It's strange because I can either trigger it on the first try, or it
> never triggers at all??

I think this could happen if you get your SWAPGS state screwed up
(so you do a mismatched swapgs) In the early days of the port I fought a 
lot with this.

One easy way to debug it is to read the GS msr early and double
check it's as expected.

-Andi
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