linux-kernel - Re: time_after

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Message-ID: <8b67d60709111610s41e1964dsc2fe21a6eca4096@mail.gmail.com>
Date:	Wed, 12 Sep 2007 00:10:19 +0100
From:	"Adrian McMenamin" <lkmladrian@...il.com>
To:	"Björn Steinbrink" <B.Steinbrink@....de>
Cc:	"Rene Herman" <rene.herman@...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 12/09/2007, Björn Steinbrink <B.Steinbrink@....de> 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.
>

No, I mean "works properly" - ie occasionally evaluates as true
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