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

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Message-ID: <20081031093804.GF30317@elte.hu>
Date:	Fri, 31 Oct 2008 10:38:04 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
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


* Steven Rostedt <rostedt@...dmis.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
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