CAS implementation may be broken

[jamie@shareable.org (Jamie Lokier)]

Toby Douglass wrote:
> Jamie Lokier wrote:
> >Note that pointer-counter is not really reliable.
> 
> [snip case]
> 
> >Of course even if the counter wraps, you still have to be 1/2^32
> >unlucky to see the same value.  But that's enough to make it unreliable.
> 
> I concur, but it's not a problem on 64-bit.

Agree, about 64-bit.  (Until we have much faster CPUs :-)

It's occurred to me that it's possible to make pointer-counter safe.

Simply, when about to wrap, send a signal to every thread.  The signal
handler should check if the thread it has interrupted is inside a
pointer-counter critical section, and if yes, force the interrupted
code to synchronise with all the other threads in another way, such as
using a mutex.

-- Jamie