linux-kernel - Re: time_after

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Date:	Wed, 12 Sep 2007 01:16:22 +0200
From:	Rene Herman <rene.herman@...il.com>
To:	Björn Steinbrink <B.Steinbrink@....de>
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 09/12/2007 01:09 AM, Björn Steinbrink wrote:
> 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.

Yes, but if jiifie wrapping was the problem, I'd expect the contrary 
behaviour with the time_after() one hitting while the > one does not.

Rene.
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