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 11:10:58 -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 Tue, Oct 18, 2011 at 10:52:22AM -0400, Seiji Aguchi wrote:
> 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.

The point I was trying to make with the BUG_ON is to catch future uses of
NMI and kmsg_dump that were implemented without understanding the
restriction we place (you have to be in a panic path for NMI to use
pstore/kmsg_dump).

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.

Cheers,
Don
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