Is RCU now possible?

[Dennis Ferguson <dennis.c.ferguson%gmail.com@localhost>]

I was wondering if it is now possible to have a full RCU
implementation in the kernel?

I realize there used to be patent issues with this.  Wikipedia
lists the following US patents:

        5,608,893
        5,727,209
        6,219,690
        6,886,162

When I look at these, however, the first patent was filed in
1993 and should now be expired while the remaining three (according
to google patents) seem to have lapsed around 2010 when IBM failed
to pay the maintenance fee to extend them.

Is there anything else?

Dennis Ferguson