linux-kernel - Re: [PATCH v11 4/5] core: Add kernel message dumper to call on oopses and panics

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Date:	Mon, 26 Oct 2009 17:13:27 +0200
From:	"Shargorodsky Atal (EXT-Teleca/Helsinki)" 
	<ext-atal.shargorodsky@...ia.com>
To:	ext Simon Kagstrom <simon.kagstrom@...insight.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Artem Bityutskiy <dedekind1@...il.com>,
	linux-mtd <linux-mtd@...ts.infradead.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Koskinen Aaro (Nokia-D/Helsinki)" <aaro.koskinen@...ia.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH v11 4/5] core: Add kernel message dumper to call on
	oopses and panics

On Mon, 2009-10-26 at 12:53 +0100, ext Simon Kagstrom wrote:
> On Mon, 26 Oct 2009 12:36:33 +0200
> "Shargorodsky Atal (EXT-Teleca/Helsinki)" <ext-atal.shargorodsky@...ia.com> wrote:
> 
> > > > I can think of a couple of way to figure it out in the module
> > > > itself, but I could not think of any clean way to do it.
> > > 
> > > This is correct, and the mtdoops driver has some provisions to handle
> > > this. First, there is a parameter to the module to specify whether
> > > oopses should be dumped at all - I added this for the particular case
> > > that someone has panic_on_oops set.
> > 
> > It takes care of most of the situations, but panic_on_oops
> > can be changed any time, even after the module is loaded.
> 
> Yes, but this parameter is settable at runtime as well by writing
> to /sys/module/mtdoops/parameters/dump_oops.
> 
> > > Second, it does not dump oopses directly anyway, but puts it in a work
> > > queue. That way, if panic_on_oops is set, it will store the panic but
> > > the oops (called from the workqueue) will not get written anyway.
> > > 
> > 
> > AFAIK, mtdoops does not put oopses in a work queue. And if by any chance
> > it does, then I think it's wrong and might lead to missed oopses, as
> > the oops might be because of the work queues themselves, or it might
> > look to the kernel like some non-fatal fault, but actually it's a
> > sign of a much more catastrophic failure - IOMMU device garbaging
> > memory, for instance.
> 
> I was referring to my patches to it, sorry. It's in the patch "[PATCH v7 5/5]:
> mtdoops: refactor as a kmsg_dumper" (as well as the parameter to dump
> oopses at all).
> 
> There are other situations which will make dumping problematic as well,
> e.g., crashes in the mtd code, so there are certainly some cases which
> will be difficult to catch. But in the panic_on_oops case or
> oops-in-interrupt, the oops won't be missed and won't be outputted
> twice for mtdoops.
> 
> 
> Anyway, I understand your problem and agree that it would be good to
> fix. Moving up crash_kexec() and the notifiers will at least fix your
> second issue. For the double-output-of-oopses, I don't see a good way
> to fix it unless relying on the module to correct it like above.
> 

How about adding KMSG_DUMP_LAST_OOPS_BEFORE_PANIC or something to the
kmsg_dump_reason enum, and making the kmsg_dump look like
kmsg_dump(panic_on_oops ? KMSG_DUMP_LAST_OOPS_BEFORE_PANIC : KMSG_DUMP_OOPS);
in oops_exit. Then let the dumpers decide what they want to do about it.
Just a thought.

And since you have no objections about moving notifiers up, it looks
like the second issue will be resolved, I believe Artem
will take care of it. :)

Thanks a lot,
Atal.

> // Simon

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