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

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Date:	Mon, 12 Oct 2009 15:39:37 +0200
From:	Simon Kagstrom <simon.kagstrom@...insight.net>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Artem Bityutskiy <dedekind1@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Koskinen Aaro \(Nokia-D/Helsinki\)" <aaro.koskinen@...ia.com>,
	linux-mtd <linux-mtd@...ts.infradead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] panic.c: export panic_on_oops

OK, I don't think we understand each other. Sorry if I'm being slow
here, please tell me if I'm misunderstanding something fundamental
below.

On Mon, 12 Oct 2009 15:15:29 +0200
Ingo Molnar <mingo@...e.hu> wrote:

> > I'm afraid I don't really see this issue. The workqueue is used to 
> > write the buffer to the mtd device if we are not in a panic or 
> > interrupt context - in which case we do it directly.
> > 
> > So it's only used when an oops is ongoing.
> 
> This fixation on 'panic' is so wrong!
> 
> 90% of the bugs users care about dont involve any panic. And even if 
> there is a panic down the line, most of the interesting messages are in 
> the stream leading up to the panic - now tucked away in that async 
> workqueue mechanism and not visible.

Well, this is what my patch [1] aims to fix. What it does is to put all
messages in a circular buffer, and when an oops or panic occurs it
writes them out. The current version only collects messages _during_ an
oops. I'll rework it with using kfifo as per Alans suggestion though.

Neither the current code nor the new patch has them stored in the work
queue during a panic though. If this happens, they will call
panic_write (if it's available) to write it out directly.

> There's two clean solutions i think:
> 
> 1) add some new "ok, there's trouble!" callback to struct console and 
>    the console driver could via that mechanism send out the _last_ 2KB 
>    (or more) of kernel log messages. Basically we can go back in time by 
>    looking at the dmesg buffer. The low level console driver does not 
>    need to 'follow' the high level console state - it only wants to 
>    print in case of trouble anyway.
>
> 2) or add buffered (flash-friendly) writes for all printk output - panic
>    and non-panic alike. This would be useful to debug suspend/resume
>    bugs for example. This would also optimize the packets of netconsole
>    output. (last i checked we sent a packet per line.)

Well, suspend/resume hangs is one of the cases which mtdoops won't
catch. But at least on NAND flash, I'd be a bit weary about logging all
printk output for fear of wearing out the flash.

> The workqueue looks wrong in both variants. If we are panic-ing (or 
> hanging, or ...) then we are halting the machine - the workqueue has no 
> chance to actually execute.

but then we are using mtd->panic_write to write it out directly, not
via the work queue.

// Simon

[1] http://patchwork.ozlabs.org/patch/35750/
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