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

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Message-ID: <Pine.LNX.4.64.0712281016290.15274@ask.diku.dk>
Date:	Fri, 28 Dec 2007 10:27:06 +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,
	yoshfuji@...ux-ipv6.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.

Isn't this only the case if timeout is a potentially large number? In this 
case, timeout may ultimately depend on a value that come from the user 
level, so I don't know what ranges are expected, but in many other cases 
one of the summands is a constant multiplied by HZ.  If the constant is 
small, then I guess that the likelihood that jiffies overflows and the 
likelihood that the sum overflows are essentially the same.  One then has 
to consider whether:

overflowed - correct </>/<=/>= small constant

is more or less desirable than

time_before/after/before_eq/after_eq(correct, overflowed_by_small_constant)

julia

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