Accessing Thread Information in kernel crash dumps with ddd+gdb

Piet Delaney pdelaney at
Fri Apr 18 17:04:03 EDT 2008

Vivek Goyal wrote:

>On Thu, Apr 17, 2008 at 05:16:55PM -0700, Piet Delaney wrote:

I was thinking over lunch that the largest risk to whacking
the task list might be a stack overflows. Perhaps a list of crash
notes could be maintained with the allocation/freeing of the
task structures. With the notes being so small it might not
be too bad just to allocate an array based on some configurable
maximum. Being able to dump say up to 1000 task notes would
be more that sufficient for our systems. The wasted memory
of allocating more memory than actually needed would be a
small cost to ensure well debugged kernels.


>>Hey Guys:
>>I've been using kgdb for a while with our 2.6.12 and now 2.6.16 kernel
>>as well as kdump/kexec with our 2.6.16 kernel. I'm a bit disappointed
>>with the visibility of local variables on the threads/tasks not currently
>>running on CPUs. Both crash, and the gdb macros that you guys wrote,
>>show the most important stuff but I'd prefer to be able to see everything
>>with gdb/ddd as I can with kgdb; including all local variables and formal
>>parameters at each stack frame.
>>A long time ago I used gdb on SunOS 4.1.4 and use to simply set $fp
>>and $sp from the saved information in the U-block to view a process.
>>I wish gdb would allow be to run your macros, btt for example, and extract
>>the stackp from task.thread.esp assign it temporally to $sp for the 
>>current task,
>>do the backtrace command and see everything. Changing $sp and $fp for a 
>>like I use to do with gdb on SunOS 4.1.4 and then using ddd+gdb to 
>>browse the
>>stack formals and locals would be nice. Just doing a 'set write on' 
>>isn't sufficient,
>>gdb wants a process and I can't see to satisfy it with simply setting 
>>the current
>>I was wondering if any of you guys have been thinking of anything like this
>>and had and hacks or ideas on how to see the locals and formals for all 
>>One thought I had was a minor hack of the kexec code to do something 
>>like your gdb macros
>>and walk thru the task list and then append a ELF Notes, like done by 
>>for each task. I have no idea if gdb has a limit on the number of 
>>elf_prstatus structures
>>that can be provided. I suppose I'd leave it a KEXEC config variable to 
>>enable this, as
>>some would argue that it's not as save as simply saving the regs for the 
>>active CPUs.
>>This would leave 'info threads' with gdb similar to 'ps' with crash and 
>>virtually identical
>>to the experience with kgdb.
>IIUC, you are suggesting that we create elf notes even for non-active
>tasks in vmcore. We should not be doing that.
>- It is not safe to traverse through task list after system has crashed.
>- We reserve the memory for elf notes at system boot. At that time we
>  have no idea how many task system will have at the time of crash.
>I think following can be a way forward for your requirement.
>- Either gdb should provide a SunOS kind of facility where one can 
>  provide stack pointer and switch the task context. ( I don't know
>  if there is already a way to do that).
>- Or one can write a user space tool, which parses original vmcore,
>  walks through task list, prepare elf notes for all the tasks and emit
>  a new vmcore which is fetched to gdb.

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