From: Akinobu Mita <amgta@yacht.ocn.ne.jp>
To: Badari Pulavarty <pbadari@us.ibm.com>,
       Hariprasad Nellitheertha <hari@in.ibm.com>
Subject: Re: [PATCH] kdump: Fix for boot problems on SMP
Date: Fri, 26 Nov 2004 02:21:49 +0900
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On Tuesday 23 November 2004 09:43, Badari Pulavarty wrote:
> gdb is not showing the stack info properly, on my saved vmcore.
> I thought vmlinux is not matching the vmcore, so I verified that
> vmcore and vmlinux matchup. But still no luck...
>
> # gdb  ../linux-2.6.9/vmlinux vmcore.2

[...]

> (gdb) bt
> #0  default_idle () at arch/i386/kernel/process.c:108
> #1  0xc04cdff8 in init_thread_union ()
> #2  0xc0101b86 in cpu_idle () at arch/i386/kernel/process.c:196
> #3  0xc04cea20 in start_kernel () at init/main.c:523
> #4  0xc0100211 in L6 () at /tmp/cch2z2jk.s:2054
> Cannot access memory at address 0x550007


I think the panic was happened on the CPU except for CPU#0.

Currently vmcore contains only CPU#0's register contents.
Therefore, GDB always shows backtrace of CPU#0.


fs/proc/vmcore.c:

static void elf_vmcore_store_hdr(char *bufp, int nphdr, int dataoff)
{
...
        /* 1 - Get the registers from the reserved memory area */
        reg_ppos = BACKUP_END + CRASH_RELOCATE_SIZE;
        read_from_oldmem(reg_buf, REG_SIZE, &reg_ppos, 0);
        elf_core_copy_regs(&prstatus.pr_reg, (struct pt_regs *)reg_buf);
        buf = storenote(&notes[0], buf);


In this place, "reg_ppos" is the pointer to the copy of relocated
crash_smp_regs[0].
kdump should save the "crash_smp_regs[**panic_cpu**]".

Or, it is better to save all crash_smp_regs[NR_CPUS].
In other words:

# readelf --note /proc/vmcore

Notes at offset 0x00000074 with length 0x0000069c:
  Owner         Data size       Description
  CORE          0x00000090      NT_PRSTATUS (prstatus structure)
  CORE          0x0000007c      NT_PRPSINFO (prpsinfo structure)
  CORE          0x00000560      NT_TASKSTRUCT (task structure)
  :
  :
  :
  ...(repeat NR_CPU times)


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