Use message-passing to implement throwTo in the RTS

This replaces some complicated locking schemes with message-passing
in the implementation of throwTo. The benefits are

 - previously it was impossible to guarantee that a throwTo from
   a thread running on one CPU to a thread running on another CPU
   would be noticed, and we had to rely on the GC to pick up these
   forgotten exceptions. This no longer happens.

 - the locking regime is simpler (though the code is about the same
   size)

 - threads can be unblocked from a blocked_exceptions queue without
   having to traverse the whole queue now.  It's a rare case, but
   replaces an O(n) operation with an O(1).

 - generally we move in the direction of sharing less between
   Capabilities (aka HECs), which will become important with other
   changes we have planned.

Also in this patch I replaced several STM-specific closure types with
a generic MUT_PRIM closure type, which allowed a lot of code in the GC
and other places to go away, hence the line-count reduction.  The
message-passing changes resulted in about a net zero line-count
difference.

1 files changed, 7 insertions, 10 deletions

diff --git a/rts/RaiseAsync.h b/rts/RaiseAsync.h
index 96eb96e10b..5137d41f5f 100644
--- a/rts/RaiseAsync.h
+++ b/rts/RaiseAsync.h
@@ -29,16 +29,13 @@ void suspendComputation (Capability *cap,
 			 StgTSO *tso, 
 			 StgUpdateFrame *stop_here);
 
-nat throwTo (Capability *cap,	         // the Capability we hold 
-	     StgTSO *source,	         // the TSO sending the exception
-	     StgTSO *target,             // the TSO receiving the exception
-	     StgClosure *exception,      // the exception closure
-	     /*[out]*/ void **out   // pass to throwToReleaseTarget()
-    );
+MessageThrowTo *throwTo (Capability *cap,      // the Capability we hold 
+                         StgTSO *source,
+                         StgTSO *target,
+                         StgClosure *exception); // the exception closure
 
-#ifdef THREADED_RTS
-void throwToReleaseTarget (void *tso);
-#endif
+nat throwToMsg (Capability *cap,
+                MessageThrowTo *msg);
 
 int  maybePerformBlockedException (Capability *cap, StgTSO *tso);
 void awakenBlockedExceptionQueue  (Capability *cap, StgTSO *tso);
@@ -52,7 +49,7 @@ interruptible(StgTSO *t)
 {
   switch (t->why_blocked) {
   case BlockedOnMVar:
-  case BlockedOnException:
+  case BlockedOnMsgThrowTo:
   case BlockedOnRead:
   case BlockedOnWrite:
 #if defined(mingw32_HOST_OS)