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

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Date:	Wed, 26 Dec 2007 12:45:32 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ray Lee <ray-lk@...rabbit.org>
CC:	Julia Lawall <julia@...u.dk>, 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.

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.

	-hpa
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