Messages in this thread | | | Date | Wed, 26 Dec 2007 12:45:32 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 1/4] fs/autofs: Use time_before, time_before_eq, etc. |
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Ray Lee wrote: > On Dec 26, 2007 7:21 AM, Julia Lawall <julia@diku.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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