On Wed, 26 Dec 2007, H. Peter Anvin wrote: > Ray Lee wrote: > > On Dec 26, 2007 7:21 AM, Julia Lawall <[EMAIL PROTECTED]> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
