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 13:48:26 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Seiji Aguchi <seiji.aguchi@....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

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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