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

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Message-ID: <2c0942db0712261158q34cddebeifceef6ceda683320@mail.gmail.com>
Date:	Wed, 26 Dec 2007 11:58:56 -0800
From:	"Ray Lee" <ray-lk@...rabbit.org>
To:	"Julia Lawall" <julia@...u.dk>
Cc:	hpa@...or.com, 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 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.
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