all mutexes -> read-write locks?

[Julian Elischer]

it has been mentioned several times that through the evolution of the
locking primitives it has come to be that mutexes and exclusively 
acquired reader-writer locks are almost the same in terms of  overhead
and that it might be a good move to define all mutexes to be
actually just that.

this would allow people to slowly go through the system, catching low
hanging fruit by converting some of the mutex operations to reader
acquisitions wherever a writer is not required, thus reducing general 
system contention.

Is there any thought on this?  Last I heard jhb had confirmed that it
was feasible..