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

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Date:	Mon, 12 Oct 2009 12:18:47 -0700
From:	Dirk Hohndel <hohndel@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Simon Kagstrom <simon.kagstrom@...insight.net>,
	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>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] panic.c: export panic_on_oops

Here's what I get for traveling and not reading mail for two days...

On Mon, 2009-10-12 at 11:45 -0700, Linus Torvalds wrote:
> 
> On Mon, 12 Oct 2009, Ingo Molnar wrote:
> > 
> > * Andrew Morton <akpm@...ux-foundation.org> wrote:
> > > 
> > > Perhaps oops_enter() is a good place to mark the start of the log, and 
> > > flush it within oops_exit().
> > 
> > Simplest would be to do the last 2K in oops_exit()? That gives the oops, 
> > and the history leading up to it. Since the blocking is 2K, the extra 
> > log output is for free.
> 
> I agree, except I don't think it should be fixed to 2k.
> 
> We should just dump as much as is "appropriate" for the dump device. It 
> might be the last 2kB, it might be 8kB, it might be 64kB. We don't know, 
> we don't care. The device may have its own per-device limits. Any extra 
> data we get from before the oops is just gravy (often there might be 
> interestign warning messages leadign up to the dump), and if the oops is 
> too big for the dump device, it's not something we can do anything about 
> anyway.

I'm working on something different but related - also using the ring
buffer and just getting as much from its tail as I am able to conserve.

The approach in my case is to write a 2D bar code to the screen and have
the user take a picture / submit the picture to kerneloops.org where it
then gets decoded back into the oops message. This is intended for
situations where you don't have access to other storage / network - or
where a picture of the screen is actually the easiest way to get to the
information.

Right now the project is slightly stalled as I am running into an
unexpected project on the decode side, but I'd love to make sure that
the core changes I'm doing integrate cleanly with this project...

> 
> So the logic should literally be something like this:
> 
>  - kernel/printk.c:
> 
> 	void dump_kmsg(void)
> 	{
> 		unsigned long len = ACCESS_ONCE(log_end);
> 		struct dump_device *dump;
> 		const char *s1, *s2;
> 		unsigned long l1, l2;
> 
> 		s1 = "";
> 		l1 = 0;
> 		s2 = log_buf;
> 		l2 = len;
> 
> 		/* Have we rotated around the circular buffer? */
> 		if (len > log_buf_len) {
> 			unsigned long pos = (len & LOG_BUF_MASK);
> 
> 			s1 = log_buf + pos;
> 			l1 = log_buf_len - pos;
> 
> 			s2 = log_buf;
> 			l2 = pos;
> 		}
> 
> 		list_for_each_entry (dump, dump_list, list) {
> 			dump->fn(s1, l1, s2, l2);
> 		}
> 	}
> 
> ie we just always give the whole buffer (as two "sections", since it's a 
> circular buffer) to the dumper, and then the dumper can decide how much of 
> those buffers it is able to dump sanely.

That's pretty close to what I do - only in my case the information then
doesn't get written to a device but instead gets compressed, encoded and
displayed on the framebuffer...

/D

-- 
Dirk Hohndel
Intel Open Source Technology Center

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