On Fri, Nov 12, 2010 at 02:30:58PM +0100, Johnny Billquist wrote: > Hmm. The thing with rwlocks though is that the actual concept is > just that you have locks that you want to grab, with the expanded > idea that you can have them at two different levels. read or write. > There is nothing inherently CAS about that. On a VAX, I can easily > implement this with native instructions, but it will not be anything > near a CAS. But now, they are more or less forced to go via a CAS > anyway, for what I'd consider no good reason.
To re-iterate: - The cheapest way to implement rwlocks on modern computers is with CAS. - rwlocks work badly in the real world. In applications where they do make sense other primitives offer a better deal. For example Linux seqlocks or distributed locks with a per-CPU component. Therefore it doesn't make sense to spend time enhancing rwlocks.
