On Fri, 11 Dec 2009, Tim Blechmann wrote:
by definition ll/sc are immune to aba problems
This is NOT correct. The sequence of operations you would need to prevent ABA is: object *tmp=head.load_linked(); object *next=tmp->next; head.store_conditional(next); This is however NOT possible because you are not allowed to make any memory access between ll and sc -- some CPUs (definitely older mips, but not ppc, unsure about many of the other architectures) clear the reservation on _any_ memory reference and will cause this sequence to always fail. Alpha has the fun requirement of a memory barrier before you are allowed to dereference "tmp", which just provides another way to kill your reservation. ll/sc only prevents "pathological" ABA races as you cannot inpsect the object being replaced. Exposing an ll/sc interface to a higher-level language is therefore unlikely to result in something usable.
most cas-architectures provide dcas
I think x86 is the rare exception, not the rule, so relying on DCAS excludes pretty much all 32-bit except for x86. Helge