Helge Bahmann wrote:
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. PPC as well. 7447, for example, will invalidate the "reservation bit" after writing to ANY memory location.
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 _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost