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Date: Tue, 22 May 2012 10:03:20 -0700
From: Tejun Heo <tj@kernel.org>
To: =?iso-8859-1?Q?J=F6rg-Volker?= Peetz <jvpeetz@web.de>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Linux 3.4 released
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On Tue, May 22, 2012 at 06:52:49PM +0200, J?rg-Volker Peetz wrote:
> I tried the above patch but was not able to see a line beginning with "XXX", not
> on the text-console nor in any log-file. After the hard-lock, I can see only the
> console-screen which now changed slightly:
> 
> BUG: Unable to handle kernel NULL pointer dereference at 0000000000000008
> IP: [<ffffffff8103ed60>] delayed_work_timer_fn+0x30/0x60

Oh, &cwq->wq is at offset 8 so cwq should have been -8.  Maybe I'm
just confused.  Can you please try the following instead?

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 5abf42f..14babfe 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1096,10 +1096,16 @@ queue_work_on(int cpu, struct workqueue_struct *wq, struct work_struct *work)
 }
 EXPORT_SYMBOL_GPL(queue_work_on);
 
+#include <linux/uaccess.h>
 static void delayed_work_timer_fn(unsigned long __data)
 {
 	struct delayed_work *dwork = (struct delayed_work *)__data;
 	struct cpu_workqueue_struct *cwq = get_work_cwq(&dwork->work);
+	unsigned long v;
+
+	if (probe_kernel_read(&v, &cwq->wq, sizeof(v)))
+		printk("XXX delayed_work_timer_fn: cwq %p, fn=%pf\n",
+		       cwq, dwork->work.func);
 
 	__queue_work(smp_processor_id(), cwq->wq, &dwork->work);
 }
--
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