linux-kernel - Re: [PATCH] panic.c: export panic_on

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Date:	Mon, 12 Oct 2009 17:14:31 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Simon Kagstrom <simon.kagstrom@...insight.net>,
	Artem Bityutskiy <dedekind1@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Koskinen Aaro (Nokia-D/Helsinki)" <aaro.koskinen@...ia.com>,
	linux-mtd <linux-mtd@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] panic.c: export panic_on_oops

* David Woodhouse <dwmw2@...radead.org> wrote:

> On Mon, 2009-10-12 at 16:26 +0200, Ingo Molnar wrote:

> > Not if the failure is say a s2ram hang that requires a power cycle. 
> > Also there are certain classes of bugs that only occur on cold boot. 
> > Plus there's the "need to unplug the battery to revive the system" 
> > class of bugs (but they are rare).
> 
> So you need to build in enough ECC to cope with the decay which 
> happens when RAM isn't being refreshed for a few seconds... :)

[ hey, i think you should line up with BIOS writers at that wall ;-) ]

> > So i think the MTD / flash stuff is powerful.
> 
> Yeah, definitely. I was just pointing out that we can actually do a 
> lot better on today's commodity hardware too.

I wish it worked on any of the 10+ x86 systems i have. Is there anyone 
who'd be interested in exploring whether warm BIOS reboots work 
_anywhere_?

A simple patch with a new (default-off) CONFIG_DEBUG_ feature that just 
puts a signature into a predictable spot in RAM, switches the reboot 
method over to warm reboot (reboot=w) and prints some friendly "yay, 
this BIOS rocks!" message if the signature is still there after a reboot 
and not zeroed out.

If that works _anywhere_ we could complete it: we could cache the dmesg 
buffer address (__log_buf[]) across reboots (and maybe the printk tail 
offset (log_end)), and that would be an _excellent_ debuggability 
feature for a large class of otherwise undebuggable crashes ...

We could use that to preserve a kernel function trace (or a branch 
execution hardware trace using BTS on Intel CPUs) across crashes, etc. 
etc.

	Ingo
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