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

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Message-ID: <20111017172229.GH5795@redhat.com>
Date:	Mon, 17 Oct 2011 13:22:29 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Seiji Aguchi <seiji.aguchi@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Matthew Garrett <mjg@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	"Chen, Gong" <gong.chen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Brown, Len" <len.brown@...el.com>,
	"Huang, Ying" <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 Mon, Oct 17, 2011 at 09:56:50AM -0700, Luck, Tony wrote:
> > All this lock busting probably isn't pretty and causes one to reflect what
> > is going on here.  But as long as we are going to keep the kmsg_dump
> > design, changes like this seem necessary to make sure the locking stays
> > sane(r) for now.
> 
> So should we let the back-end know that locks have been busted?  I worry
> that we'll get through the pstore layer by blasting away at any locks that
> get in our way - but then the backend (which may well have been written on
> the assumption that pstore serialized calls to it) will not do anything
> useful (and may cause its own hang).

I was kinda alluding to something like that, when I said we probably need
follow on patches to clean up the backend.  But maybe it would be smarter
to set a flag in pstore to let the backend know we busted the locks
instead of letting them do panic checks themselves.  I agree with your
concerns.

I really can't think of a good overall solution other than slowly
step-by-step untangle things by making subtle changes, this round being
the serialization of kmsg_dump().

My brain is to small to figure this out.  I keep thinking about making an
ascii art diagram to see all the various paths and their contexts, but
then I keep finding more interesting things to do. :-)

Perhaps we should do that and make some rules about what the back-end can
and can not do when plugging into kmsg_dump.

Cheers,
Don

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