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, 2 insertions, 3 deletions

diff --git a/rts/HeapStackCheck.cmm b/rts/HeapStackCheck.cmm
index b516ef2c09..a528a3f22e 100644
--- a/rts/HeapStackCheck.cmm
+++ b/rts/HeapStackCheck.cmm
@@ -631,9 +631,8 @@ INFO_TABLE_RET( stg_block_throwto, RET_SMALL, P_ unused, P_ unused )
 
 stg_block_throwto_finally
 {
-#ifdef THREADED_RTS
-    foreign "C" throwToReleaseTarget (R3 "ptr");
-#endif
+    // unlock the throwto message
+    unlockClosure(StgTSO_block_info(CurrentTSO), stg_MSG_THROWTO_info);
     jump StgReturn;
 }