linux-kernel - Re: time_after

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Message-ID: <20070911230940.GA7070@atjola.homenet>
Date:	Wed, 12 Sep 2007 01:09:40 +0200
From:	Björn Steinbrink <B.Steinbrink@....de>
To:	Rene Herman <rene.herman@...il.com>
Cc:	Adrian McMenamin <lkmladrian@...il.com>,
	linux-kernel@...r.kernel.org, linuxsh-dev@...ts.sourceforge.net,
	Paul Mundt <lethal@...ux-sh.org>
Subject: Re: time_after - what on earth???

On 2007.09.12 00:19:09 +0200, Rene Herman wrote:
> On 09/12/2007 12:15 AM, Adrian McMenamin wrote:
>
>> On 11/09/2007, Rene Herman <rene.herman@...il.com> wrote:
>>> On 09/12/2007 12:05 AM, Adrian McMenamin wrote:
>>>
>>>> OK, why does this line occasionally return true:

What exactly is "occassionally"?  Does it happen more than once per
boot? If not, and it happens after a certain time after booting, it
might be wrapping of the jiffie counter (see below).

>>>>
>>>>       if ((maple_dev->interval > 0) && (jiffies >maple_dev->when))
>>>>
>>>> while this one never does (no other changes made):
>>>>
>>>> if  ((maple_dev->interval > 0) && (time_after(jiffies, 
>>>> maple_dev->when)))
>>> Is maple_dev->when an unsigned long?
>>>
>> Yes. Does that make a difference?
>
> If it had been a signed type, it could've wrapped to something you didn't 
> expect, explaining the difference at least...
>
> With an unsigned long, the only diference should be that time_after() deals 
> with jiffie wrapping which I assume is not an actual problem here. I'll 
> retreat into the shades again... ;-(

If "occasionally" is limited to once per boot, it might be jiffie
wrapping. IIRC jiffies are initialized so that they wrap after about 5
minutes of uptime to reveal such bugs without forcing you to wait for
ages just to have the counter wrap for the first time.

Björn
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