linux-kernel - Re: [PATCH -v2] ring-buffer: add paranoid checks for loops

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Message-ID: <alpine.DEB.1.10.0810310958400.13290@gandalf.stny.rr.com>
Date:	Fri, 31 Oct 2008 10:00:09 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH -v2] ring-buffer: add paranoid checks for loops


On Fri, 31 Oct 2008, Ingo Molnar wrote:
> 
> > +	/*
> > +	 * This should normally only loop twice. But because the
> > +	 * start of the reader inserts an empty page, it causes
> > +	 * a case where we will loop three times. There should be no
> > +	 * reason to loop four times (that I know of).
> > +	 */
> > +	if (unlikely(paranoid > 2)) {
> > +		RB_WARN_ON(cpu_buffer, 1);
> > +		reader = NULL;
> > +		goto out;
> > +	}
> > +	paranoid++;
> 
> ok, the explanations look nice now.
> 
> A small nit - the above comment suggests that looping 4 times is the 
> anomaly - still the test is for paranoid > 2 ?

Yes, that's because the variable started at 0. So > 2 really means the 
loop iterated more than 3.

My last patch changed it to be a bit easier to understand...

   if (unlikely(++nr_loops > 3)) {

-- Steve

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