Re: oops in :snd_pcm_oss:resample_expand+0x19c/0x1f0

From: Jean-Marc Saffroy
Date: Mon Sep 25 2006 - 17:02:36 EST

Hi Vivek,

Thanks for all the good news on the future of kdump, it's nice to know that people are working on improving the user experience.

On Mon, 25 Sep 2006, Vivek Goyal wrote:

Oh and I wish I could use gdb on a kdump core. :-)

Currently we can use gdb but only for linearly mapped region. You are right its just a matter of re-generating the elf headers and remap the vmalloc areas to enable module debugging in gdb. I can not do it after the crash so probably the best place would be do it in user space. A program can read /proc/vmcore and regenerate the headers for enabling module debugging with gdb.

I assume that "after the crash" means "in the kernel crash handler", AFAICT the current dump from vmcore has all what's needed.

Hmm.. Crash vs gdb is an interesting issue. I have not used gdb very extensively on core dumps, but with my limited experiece, I found "crash" to be more friendly.

One thing I like *very much* in gdb is its ability to display function params and local variables in any stack frame, and I haven't found out how to do it with crash.

Crash has got so many in-built commands tailored for kernel debugging and gdb lacks all those. Yes, we can write gdb scripts to implement those, but last time Alaxender Nyberg wrote few gdb scripts to dump all the threads and it was so slow.

I agree that gdb is sometimes very slow, but maybe it's easier to optimize gdb than to make crash smarter?

For this particular problem (listing threads), the real fix would be to add the PT_NOTE entry that each thread deserves, then gdb would let you do "info threads" instead, and dump nice backtraces of each.

Look at Documentation/kdump/gdbmacros.txt

Hmmm, these need an update, they no longer work with 2.6.18. But I have an idea of how slow they can be, having tried a few things myself.

So what's issue with crash? Is it just a matter of being more familiar with gdb or gdb has got advantages over "crash" when it comes to kernel debugging?

Oh I am certainly biased towards gdb :-) but function params and local vars are very useful when debugging.

Of course crash is still a very useful tool, and until we can really use gdb on kdumps (which requires some work), it will remain the best option we have. Heck, it even impressed Andrew Morton! ;-)


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