On Thu, Nov 29, 2012 at 08:20:03PM +0100, Andi Kleen wrote: > > The trick is that we don't watch for the refcount hitting 0 until we're > > shutting down - so this only works if you keep track of your initial > > refcount. As long as we're not shutting down, we know the refcount can't > > hit 0 because we haven't released the initial refcount. > > This seems dangerous to me: assume you have one CPU which always > does get and another does put. So there may be 2^32 such operations > without a kill and you wrap for real in a way that does not get > corrected. I don't know how to write a proof that it works (and I should... I haven't done any real math in ages, argh) but try working out some examples to see what happens: cpu 0 does 2^32 gets, cpu 1 does 2^32 - 1 puts, actual ref should be 1: cpu 0 ref: 0 cpu 1 ref: 1 (it started at 0, and subtracted 1 2^32 - 1 times) cpu 0 does 2^32 + 1 gets, cpu 1 does 2^32 puts, again ref should be 1: cpu 0 ref: 1 cpu 1 ref: 0 There's some kind of symmetry going on here, and if I'd been awake more in college I could probably say exactly why it works, but it does. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
