linux-kernel - Re: [PATCH] timer: Fix jiffies wrap behavior of round

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Message-Id: <20130529160150.f0a498d188dc790d018200e9@linux-foundation.org>
Date:	Wed, 29 May 2013 16:01:50 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Bart Van Assche <bart.vanassche@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...radead.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Joe Perches <joe@...ches.com>
Subject: Re: [PATCH] timer: Fix jiffies wrap behavior of round_jiffies*()

On Tue, 21 May 2013 20:43:50 +0200 Bart Van Assche <bart.vanassche@...il.com> wrote:

> Make sure that the round_jiffies*() functions return a time that is
> in the future when the jiffies counter is about to wrap.

Actually "when the jiffies counter has recently wrapped".

I assume this was found by inspection?

> @@ -149,9 +149,7 @@ static unsigned long round_jiffies_common(unsigned long j, int cpu,
>  	/* now that we have rounded, subtract the extra skew again */
>  	j -= cpu * 3;
> -	if (j <= jiffies) /* rounding ate our timeout entirely; */

This used to be a very common bug - we've weeded out many instances but
I'm sure more still remain.  We could perhaps have a checkpatch rule
which looks for comparisons against jiffes (and any other
time-measuring variables we can detect) and tells people to use
time_after() and friends, or time_is_*_jiffies().

> -		return original;
> -	return j;
> +	return time_is_after_jiffies(j) ? j : original;
>  }

Your email client mangles patches, btw.
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