linux-kernel - Re: [PATCH/RFC v5 4/5]: core: Add dump device to call on oopses and panics

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Date:	Mon, 30 Nov 2009 11:40:55 +0200
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Jörn Engel <joern@...fs.org>
Cc:	Simon Kagstrom <simon.kagstrom@...insight.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-mtd <linux-mtd@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Koskinen Aaro (Nokia-D/Helsinki)" <aaro.koskinen@...ia.com>,
	Ingo Molnar <mingo@...e.hu>,
	David Woodhouse <dwmw2@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH/RFC v5 4/5]: core: Add dump device to call on oopses
 and panics

On Mon, 2009-11-30 at 10:35 +0100, Jörn Engel wrote:
> On Mon, 30 November 2009 10:51:58 +0200, Artem Bityutskiy wrote:
> > 
> > How about this patch on top of the existing one (untested):
> > 
> > +	/*
> > +	 * Have we ever rotated around the circular buffer? If we never did,
> > +	 * we have to have zeroes at the end.
> > +	 */
> > +	if (log_buf[end]) {
> > +		s1 = log_buf + end;
> > +		l1 = log_buf_len - end;
> > +	} else {
> > +		s1 = "";
> > +		l1 = 0;
> 
> So now you are assuming that a) the buffer is initially zeroed and b)
> noone ever writes NUL to it.  Is that correct?

a) seems to be true because the buffer is either a static array or a
bootmem alloc, which seems to memzero the buffers it returns, at least
AFAICS. But I did not test this.

vs b). well, the printk ring buffer should contain ASCII, so I assumed
binary zeroes should not be possible there.

> I'm not sure whether those assumptions are valid.  If they are, then
> this will obviously work.  Otherwise we can just always assume the
> wrapped case.

Of course someone who has more knowlege about the printk buffer should
comment on this.

The other alternative I was thinking about was to introduce a boolean
flag, and set it to one as soon as 'lon_end' becomes larger than
'log_buf_len'.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

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