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 10:53:31 +0100
From:	Simon Kagstrom <simon.kagstrom@...insight.net>
To:	dedekind1@...il.com
Cc:	Jörn Engel <joern@...fs.org>,
	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, 30 Nov 2009 11:40:55 +0200
Artem Bityutskiy <dedekind1@...il.com> wrote:

> > > +	/*
> > > +	 * 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.

True as far as I can see.

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

Yes, if this would be printed with printk it sounds like a bug to me.

I've tested your patch, and it seems to work fine both 1) before the
buffer is filled, 2) when the buffer has wrapped and 3) when log_end
wraps (although that part was a quick hack).

So

Tested-by: Simon Kagstrom <simon.kagstrom@...insight.net>

// Simon
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