Date: Wed, 12 Sep 2007 01:09:40 +0200
From: =?iso-8859-1?Q?Bj=F6rn?= Steinbrink <B.Steinbrink@gmx.de>
To: Rene Herman <rene.herman@gmail.com>
Cc: Adrian McMenamin <lkmladrian@gmail.com>, linux-kernel@vger.kernel.org,
       linuxsh-dev@lists.sourceforge.net, Paul Mundt <lethal@linux-sh.org>
Subject: Re: time_after - what on earth???
Message-ID: <20070911230940.GA7070@atjola.homenet>
References: <8b67d60709111505u61771a1do74c82a55430ec214@mail.gmail.com> <46E7128D.1080006@gmail.com> <8b67d60709111515m5b45c57emef5cc2f74795ed7a@mail.gmail.com> <46E7145D.10907@gmail.com>
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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@gmail.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.

Bj?rn
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