linux-kernel - Re: [PATCH 1/4] fs/autofs: Use time_before, time_before

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Message-ID: <Pine.LNX.4.64.0712270805370.6952@ask.diku.dk>
Date:	Thu, 27 Dec 2007 08:08:53 +0100 (CET)
From:	Julia Lawall <julia@...u.dk>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ray Lee <ray-lk@...rabbit.org>, autofs@...ux.kernel.org,
	linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH 1/4] fs/autofs: Use time_before, time_before_eq, etc.

On Wed, 26 Dec 2007, H. Peter Anvin wrote:

> Ray Lee wrote:
> > On Dec 26, 2007 7:21 AM, Julia Lawall <julia@...u.dk> wrote:
> > > -               if (jiffies - ent->last_usage < timeout)
> > > +               if (time_before(jiffies, ent->last_usage + timeout))
> > 
> > I don't think this is a safe change? subtraction is always safe (if
> > you think about it as 'distance'), addition isn't always safe unless
> > you know the range. The time_before macro will expand that out to
> > (effectively):
> > 
> >   if ( (long)(ent->last_usage + timeout) - (long)(jiffies) < 0 )
> > 
> > which seems to introduce an overflow condition in the first term.
> > 
> > Dunno, I may be wrong (happens often), but at the very least what
> > you've transformed it into is no longer obviously correct, and so it's
> > not a great change.
> 
> Indeed.  The bottom form will have overflow issues at time
> jiffies_wraparound/2, whereas the top form will have overflow issues only near
> jiffies_wraparound/1.

OK, so it seems like it is not such a good idea.

There are, however, over 200 files that contain calls to the various time 
functions that follow this pattern, eg:

arch/arm/kernel/ecard.c:563
if (!last || time_after(jiffies, last + 5*HZ)) {

Perhaps they should be coverted to use a subtraction as well?

julia

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