linux-kernel - Re: [PATCH 26/28] kdb,panic,debug_core: Allow the debug core to receive a panic before smp_send

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Date:	Fri, 12 Feb 2010 17:43:39 -0600
From:	Jason Wessel <jason.wessel@...driver.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
	mingo@...e.hu
Subject: Re: [PATCH 26/28] kdb,panic,debug_core: Allow the debug core to receive
 a panic before smp_send_stop()

Andrew Morton wrote:
> On Fri, 12 Feb 2010 16:35:41 -0600 Jason Wessel <jason.wessel@...driver.com> wrote:
>
>   
>> It is highly desirable to trap into kdb on panic,
>>     
>
> s/kdb/kgdb/
>
> But is it necessary to do it in-kernel?  Why not just put a
> breakpoint at panic(), add that to your .gdbinit?
>
>   

If you don't mind consuming a breakpoint that is ok.  Certainly there is
also the possibility of making this configurable as well.

For now I will omit this patch from any kind of future pull request
while we hash out what makes sense.

>> diff --git a/kernel/panic.c b/kernel/panic.c
>> index c787333..428d15b 100644
>> --- a/kernel/panic.c
>> +++ b/kernel/panic.c
>> @@ -66,10 +66,13 @@ NORET_TYPE void panic(const char * fmt, ...)
>>  	 */
>>  	preempt_disable();
>>  
>> -	bust_spinlocks(1);
>>  	va_start(args, fmt);
>>  	vsnprintf(buf, sizeof(buf), fmt, args);
>>  	va_end(args);
>> +
>> +	atomic_notifier_call_chain(&panic_notifier_list, 0, buf);
>> +
>> +	bust_spinlocks(1);
>>  	printk(KERN_EMERG "Kernel panic - not syncing: %s\n",buf);
>>  #ifdef CONFIG_DEBUG_BUGVERBOSE
>>  	dump_stack();
>> @@ -91,8 +94,6 @@ NORET_TYPE void panic(const char * fmt, ...)
>>  	 */
>>  	smp_send_stop();
>>  
>> -	atomic_notifier_call_chain(&panic_notifier_list, 0, buf);
>> -
>>  	bust_spinlocks(0);
>>  
>>  	if (!panic_blink)
>>     
>
> So the notifier call now happens before all the printks and the kexec
> and kmsg_dump handling.  What effect does this have upon the code which
> implements kexec and kmsg_dump?
>
>   

I certainly don't want to break kexec or alter any behavior, does that
mean kgdb / kdb should hook the kexec for notification?

I think ideally it is a end user's preference as to if they want in via
kexec or the kernel debugger.  Calling the smp_send_stop() prior to the
notifier was a death sentence for the kernel debugger. 

Perhaps I can move the notifier before smp_send_stop()?

Jason.
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