linux-kernel - Re: [PATCH] kexec: set prstatus.pr_pid to cpu id when current->pid is 0

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Date:	Tue, 3 Aug 2010 15:44:35 +0800
From:	Hui Zhu <teawater@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	WANG Cong <xiyou.wangcong@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Simon Kagstrom <simon.kagstrom@...insight.net>,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	gdb@...rceware.org
Subject: Re: [PATCH] kexec: set prstatus.pr_pid to cpu id when current->pid is 
	0

On Tue, Aug 3, 2010 at 15:37, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Hui Zhu <teawater@...il.com> writes:
>
>> Hi,
>>
>> I found that from gdb 7.1 to gdb-cvs-head cannot analyze the core file
>> that get from kdump.
>> What I got:
>> [New <main task>]
>> [New Thread 2719]
>> ../../src/gdb/thread.c:884: internal-error: switch_to_thread:
>> Assertion `inf != NULL' failed.
>> A problem internal to GDB has been detected,
>> further debugging may prove unreliable.
>> Quit this debugging session? (y or n)
>> That is because:
>>  objdump -h ./vmcore
>>
>> ./vmcore:     file format elf64-x86-64
>>
>> Sections:
>> Idx Name          Size      VMA               LMA               File off  Algn
>>   0 note0         00000a48  0000000000000000  0000000000000000  00000238  2**0
>>                   CONTENTS, READONLY
>>   1 .reg/0        000000d8  0000000000000000  0000000000000000  000002bc  2**2
>>                   CONTENTS
>>   2 .reg          000000d8  0000000000000000  0000000000000000  000002bc  2**2
>>                   CONTENTS
>>   3 .reg/2719     000000d8  0000000000000000  0000000000000000  00000420  2**2
>>                   CONTENTS
>>   4 .reg/0        000000d8  0000000000000000  0000000000000000  00000584  2**2
>>                   CONTENTS
>>   5 .reg/0        000000d8  0000000000000000  0000000000000000  000006e8  2**2
>>                   CONTENTS
>> Each of reg/n is a cpu core note.  It will be a GDB thread.  n is the
>> prstatus.pr_pid that will be the thread lwpid.  Because the 3 threads
>> pid is same, so GDB get error.
>>
>> current->pid is 0 because this cpu is in idle.  So I add a check, set
>> prstatus.pr_pid to cpu id when current->pid is 0.  Then GDB work OK
>> with the core.
>
> That is a gdb limitation.  It looks to me like applying this patch will
> loose information, and give you no guarantee that prstatus.pr_pid will
> not equal 0.
>
> If you want to change something please do it in a post processing tool.
>
> Eric

Equal 0 is not a bug, the trouble is a lot of core's pid is same.

This is what gdb say:
/* Found an old thread with the same id.  It has to be dead,
       otherwise we wouldn't be adding a new thread with the same id.
       The OS is reusing this id --- delete it, and recreate a new
       one.  */

Hui

>
>
>> Thanks,
>> Hui
>>
>> Signed-off-by: Hui Zhu <teawater@...il.com>
>> ---
>>  kernel/kexec.c |    5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> --- a/kernel/kexec.c
>> +++ b/kernel/kexec.c
>> @@ -1191,7 +1191,10 @@ void crash_save_cpu(struct pt_regs *regs
>>       if (!buf)
>>               return;
>>       memset(&prstatus, 0, sizeof(prstatus));
>> -     prstatus.pr_pid = current->pid;
>> +     if (current->pid)
>> +             prstatus.pr_pid = current->pid;
>> +     else
>> +             prstatus.pr_pid = cpu;
>>       elf_core_copy_kernel_regs(&prstatus.pr_reg, regs);
>>       buf = append_elf_note(buf, KEXEC_CORE_NOTE_NAME, NT_PRSTATUS,
>>                             &prstatus, sizeof(prstatus));
>
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