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Date: Fri, 31 Oct 2008 10:38:04 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
       Peter Zijlstra <peterz@infradead.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH -v2] ring-buffer: add paranoid checks for loops
Message-ID: <20081031093804.GF30317@elte.hu>
References: <alpine.DEB.1.10.0810291439460.13214@gandalf.stny.rr.com> <20081030184552.GC17822@elte.hu> <alpine.DEB.1.10.0810302314460.26352@gandalf.stny.rr.com>
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* Steven Rostedt <rostedt@goodmis.org> 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 ?

> +	int paranoid = 0;

another small nit: i'd suggest to rename 'paranoid' to 'nr_loops' or 
'nr_iterations' or so. It is the _condition_ that signals paranoia, 
not the variable in itself - making the current patch look a bit 
weird.

>   again:
> +	/*
> +	 * We repeat when a timestamp is encountered. It is possible
> +	 * to get multiple timestamps from an interrupt entering just
> +	 * as one timestamp is about to be written. The max times
> +	 * that this can happen is the number of nested interrupts we
> +	 * can have.  10 should be more than enough.
> +	 */
> +	if (unlikely(paranoid > 10)) {
> +		RB_WARN_ON(cpu_buffer, 1);
> +		return NULL;

s/10 should be more than enough/Nesting higher than 10 is clearly 
anomalous/

> +	/*
> +	 * We repeat when a timestamp is encountered. It is possible
> +	 * to get multiple timestamps from an interrupt entering just
> +	 * as one timestamp is about to be written. The max times
> +	 * that this can happen is the number of nested interrupts we
> +	 * can have.  10 should be more than enough.
> +	 */
> +	if (unlikely(paranoid > 10)) {
> +		RB_WARN_ON(cpu_buffer, 1);
> +		return NULL;

ditto.

	Ingo