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Re: mutexes, locks and so on...

On Thu, Nov 11, 2010 at 06:35:55PM +0100, Johnny Billquist wrote:
> Hmm. Hard to argue about this. While I think it's nice if we try to
> keep the kernel agnostic, the user api is not something I'm arguing
> about changing. But it would be nice if we could leave it possible
> to do things in different ways when it's not really the effect of a
> CAS that is needed, but we'd still like to keep it in a way that
> allowed the compiled code to be as nice and efficient as possible.

A CAS is pretty much the standard low-level primitive in the literature.
Either CAS or LL/SC are available in pretty much all non-ancient CPUs
and CAS can be emulated with LL/SC easily. If you really want to spend
time on reducing the overhead for VAX, implement ras(9) like behavior
for the kernel. That requires changing all trap entry points OR all trap
exit points to check if the return address is inside a special range
(e.g. a page that implements CAS and maybe a few other atomic ops) and
do a second level lookup if it is to backout to the start of the
routine. The problem with this approach is that it creates very nasty
bugs if you skip even a single instance.


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