Unreferenced lock variables

[Simon Wright <simon@pogner.demon.co.uk>]

I've been looking at using a 'control by allocation' technique (I'm
sure there's a more standard name, but I can't find it at the moment:
a resource is encapsulated in a type such that declaring an object of
the type allocates the resource).

For example,

  procedure Mark (R : in out Synchronized_Unbounded_Ring) is
    Lock : BC.Support.Synchronization.Write_Lock (R.The_Monitor);
  begin
    Mark (Unbounded_Ring (R));
  end Mark;

where the initialization of Lock calls the appropriate operations on
R.The_Monitor (which is a class-wide access ..) to ensure exclusion,
in this case for a Writer.

Now, GNAT quite properly warns me that Lock is unused; so, having been
bitten this way once, I cast around and found that inserting

    pragma Volatile (Lock);

will eliminate the warning.

However, it doesn't seem to be as legitimate a use as in the recent
thread on Handling Addressing Errors; is there a better way? pragma
Import (Ada, Lock) perhaps? or should I look for a different style?