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

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Date:	Tue, 18 Oct 2011 10:52:22 -0400
From:	Seiji Aguchi <seiji.aguchi@....com>
To:	Don Zickus <dzickus@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Chen, Gong" <gong.chen@...el.com>
CC:	"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,

>> afacit this assumes that (reason == KMSG_DUMP_PANIC) if in_nmi().  Is
>> that always the case, and will it always be the case in the future?
>

Currently, when kernel is in nmi context and kmsg_dump() are called, its reason is always "KMSG_DUMP_PANIC".

>
>Perhaps a 'BUG_ON(in_nmi() && reason != KMSG_DUMP_PANIC)'?

I don't think BUG_ON() is needed.

If someone would like to log messages in the case of "in_nmi() && reason != KMSG_DUMP_PANIC",
he/she will add a new trigger as follows.

We can discuss implementation of kmsg_dump() at that time.
But I can't see any use case now.

	371 static notrace __kprobes void 
	372 unknown_nmi_error(unsigned char reason, struct pt_regs *regs) 
	373 {
	374         if (notify_die(DIE_NMIUNKNOWN, "nmi", regs, reason, 2, SIGINT) == 
	375                         NOTIFY_STOP) 
	376                 return; 
	377 #ifdef CONFIG_MCA 
	378         /* 
	379          * Might actually be able to figure out what the guilty party 
	380          * is: 
	381          */ 
	382         if (MCA_bus) { 
	383                 mca_handle_nmi(); 
	384                 return; 
	385         } 
	386 #endif 
	387         pr_emerg("Uhhuh. NMI received for unknown reason %02x on CPU %d.\n", 
	388                  reason, smp_processor_id()); 
	389  
	390         pr_emerg("Do you have a strange power saving mode enabled?\n"); 
	391         if (unknown_nmi_panic || panic_on_unrecovered_nmi) 
	392                 panic("NMI: Not continuing"); 
	393  
	394         pr_emerg("Dazed and confused, but trying to continue\n");
		    *kmsg_dump(NMI);*
	395 }

>>
>> I felt that the spin_trylock() approach was less horrid than this.  I
>> assume that the new approach will cause lockdep to go berzerk?
>
>Heh.  Good point.  That is probably a good test case.  Though finding a
>working GHES implementation in the firmware isn't easy these days, making
>it hard to test. :-/

Thank you for supporting my patch.
But I can resend another patch below in accordance with Chen's request.
We can avoid potential deplock issue with this patch.
And this is simpler than trylock approach.

> <snip>
> If(!panic)
> 	spin_lock_irqsave();
>
>       .
>       .
> If(!panic)
> 	spin_unlock_restore();
> <snip>

Seiji
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