linux-kernel - Re: State of kgdb on x86-64

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Date:	Tue, 15 Jan 2008 19:44:24 +0100
From:	Jan Kiszka <jan.kiszka@....de>
To:	Jason Wessel <jason.wessel@...driver.com>
CC:	Jan Kiszka <jan.kiszka@...mens.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: State of kgdb on x86-64

Jason Wessel wrote:
> Jan Kiszka wrote:
>> Jason Wessel wrote:
>>   
>>> It was working at the point that I tested it with the 2.6.24-rc5 on
>>> x86_64.  However I suspect my kernel config may differ drastically from
>>> what you are using.
>>>
>>> Without any other context provided than the generic message, it is hard
>>> to know what might have happened. 
>>>     
>> Here is the promised .config. I could also dig out the backtrace of the
>> panic as kgdb sees it if that helps, just let me know.
>>
>> Jan
>>
>>   
> The backtrace might be very telling as to what happened.  More
> information is always better than less :-)
> 

My primary test box is again out of reach, but meanwhile I was able to
reproduce some kind of problem under QEMU - that one at least is
triggered by SMP. With only one CPU -> all apparently fine. Once booting
QEMU with "-smp 2" -> this happens:

(gdb) tar remote /dev/pts/6
Remote debugging using /dev/pts/6
Not all CPUs have been synced for KGDB
breakpoint () at kernel/kgdb.c:1895
1895            wmb(); /* Sync point after breakpoint */
(gdb) c
Continuing.
Not all CPUs have been synced for KGDB
[New Thread 32769]

Program received signal SIGFPE, Arithmetic exception.
[Switching to Thread 32769]
0xffffffff8020adb7 in default_idle () at include/asm/irqflags_64.h:140
140             __asm__ __volatile__("sti; hlt" : : : "memory");
(gdb) bt
#0  0xffffffff8020adb7 in default_idle () at include/asm/irqflags_64.h:140
#1  0xffffffff8020ae65 in cpu_idle () at arch/x86/kernel/process_64.c:225
#2  0xffffffff8021ccb9 in start_secondary () at arch/x86/kernel/smpboot_64.c:375
#3  0x0000000000000000 in ?? ()
(gdb)                                                                                     

The problem seems to be related to continuing SMP boxes. I'm able to
boot my box up if I leave kgdb unattached. But when I then later attach
and continue execution, I get the same crash. Any ideas what goes wrong,
any suggestion where to start digging? Maybe at "Not all CPUs have been
synched"?

Jan

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