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

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Date:	Fri, 28 Dec 2007 10:39:01 +0900 (JST)
From:	YOSHIFUJI Hideaki / 吉藤英明 
	<yoshfuji@...ux-ipv6.org>
To:	julia@...u.dk
Cc:	hpa@...or.com, ray-lk@...rabbit.org, autofs@...ux.kernel.org,
	linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
	yoshfuji@...ux-ipv6.org
Subject: Re: [PATCH 1/4] fs/autofs: Use time_before, time_before_eq, etc.

In article <Pine.LNX.4.64.0712270805370.6952@....diku.dk> (at Thu, 27 Dec 2007 08:08:53 +0100 (CET)), Julia Lawall <julia@...u.dk> says:

> 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?

No, use time_after() etc., unless you have very good reason not using them.
And above is not a good reason at all.
Frequency is not a problem.  If we have longer timeout which could
result in wrap-around, we must use another method, e.g. 64bit jiffies,
anyway.

--yoshfuji
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