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, 1 insertions, 2 deletions

diff --git a/rts/STM.c b/rts/STM.c
index ed5a7224ef..be61538434 100644
--- a/rts/STM.c
+++ b/rts/STM.c
@@ -352,8 +352,7 @@ static StgBool watcher_is_tso(StgTVarWatchQueue *q) {
 
 static StgBool watcher_is_invariant(StgTVarWatchQueue *q) {
   StgClosure *c = q -> closure;
-  StgInfoTable *info = get_itbl(c);
-  return (info -> type) == ATOMIC_INVARIANT;
+  return (c->header.info == &stg_ATOMIC_INVARIANT_info);
 }
 
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