Chris Thomasson wrote:
The basic technique for 'copying' is as follows:
rc* copy(rc **sloc, int count) { 1: load ptr from *sloc if ptr is null goto 2 lock spinlock associated with ptr re-load ptr from *sloc compare w/ previous load if not equal unlock and goto 1 XADD ptr's with count * if the previous value was less than 1 unlock and goto 1 unlock 2: return ptr }
I thought that that might be the case. Won't you need to undo the XADD before unlocking and retrying, though? That, or use CAS. The decrement thread may not care, but if another increment thread gets in first, bad things happen; or am I missing something else?
IMHO, anytime you can reduce the number of atomic operations and/or memory barriers is a plus...
Maybe... Given a choice between the two, I would prefer to somehow eliminate the spinlock in 'copy', though. :-)