On Wed, 26 Dec 2007, H. Peter Anvin wrote: > Ray Lee wrote: > > On Dec 26, 2007 7:21 AM, Julia Lawall <julia@xxxxxxx> 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. OK, so it seems like it is not such a good idea. There are, however, over 200 files that contain calls to the various time functions that follow this pattern, eg: arch/arm/kernel/ecard.c:563 if (!last || time_after(jiffies, last + 5*HZ)) { Perhaps they should be coverted to use a subtraction as well? julia - To unsubscribe from this list: send the line "unsubscribe kernel-janitors" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html