linux-kernel - RE: [RFC][PATCH -next] make pstore/kmsg_dump run after stopping other cpus in panic path

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Date:	Thu, 20 Oct 2011 14:39:36 -0400
From:	Seiji Aguchi <seiji.aguchi@....com>
To:	Don Zickus <dzickus@...hat.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"Chen, Gong" <gong.chen@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Luck, Tony" <tony.luck@...el.com>,
	Matthew Garrett <mjg@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	"len.brown@...el.com" <len.brown@...el.com>,
	"ying.huang@...el.com" <ying.huang@...el.com>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	"hughd@...omium.org" <hughd@...omium.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"jmorris@...ei.org" <jmorris@...ei.org>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"namhyung@...il.com" <namhyung@...il.com>,
	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>,
	Satoru Moriya <satoru.moriya@....com>
Subject: RE: [RFC][PATCH -next] make pstore/kmsg_dump run after stopping
 other cpus in panic path

Hi,

>I am confused.  Did you mean 'panic' kernel parameter is _not_ set?
>

I meant that  if panic parameter _is_ set (like panic=1), 
emergency_restart() will be called in panic path.
And then, kmsg_dump(KMSG_DUMP_EMERG) will be called.

        <snip>
	60 NORET_TYPE void panic(const char * fmt, ...)  
	61 {
	.
	.
	123         if (panic_timeout != 0) { 
	124                 /* 
	125                  * This will not be a clean reboot, with everything 
	126                  * shutting down.  But if there is a chance of 
	127                  * rebooting the system it will be rebooted. 
	128                  */ 
	129                 emergency_restart(); 
	130         }
	<snip>

>Actually, BUG_ON is probably overkill, perhaps a WARN_ON would be more
>appropriate.

I don't think both BUG_ON and WARN_ON are appropriate.
kmsg_dump(KMSG_DUMP_EMERG) should be called successfully in panic path.

>I wonder if it would be smarter to split your patch in half.  The part
>where you move kmsg_dump to below stop_cpus is probably non-controversial.
>That might make it sooner than the more controversial bust_spinlock
>pieces.

Thank you for your suggestion. I will split my patch.

Seiji

>On Thu, Oct 20, 2011 at 11:13:26AM -0400, Seiji Aguchi wrote:
>> Don,
>>
>> >That is part of the wider problem with kmsg_dump that Vivek talks about
>> >with me, is that it is just a giant hook in the panic path with limited
>> >auditing.  So we need to explicit set our expectations with BUG_ONs/WARNs
>> >otherwise we might get bit later by them.
>>
>> I found an issue while developing a patch v2.
>>
>> We can't call BUG_ON(in_nmi() && reason != KMSG_DUMP_PANIC)
>> because kmsg_dump(KMSG_DUMP_EMERG) is called in NMI context if "panic" kernel parameter is set.
>
>I am confused.  Did you mean 'panic' kernel parameter is _not_ set?
>
>Actually, BUG_ON is probably overkill, perhaps a WARN_ON would be more
>appropriate.
>
>>
>> I will keep doing research bust_spinlock() or vprintk() so that lockdep checking works and we can avoid any deadlocks.
>
>I wonder if it would be smarter to split your patch in half.  The part
>where you move kmsg_dump to below stop_cpus is probably non-controversial.
>That might make it sooner than the more controversial bust_spinlock
>pieces.
>
>Cheers,
>Don
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