New implementation of BLACKHOLEs

This replaces the global blackhole_queue with a clever scheme that
enables us to queue up blocked threads on the closure that they are
blocked on, while still avoiding atomic instructions in the common
case.

Advantages:

 - gets rid of a locked global data structure and some tricky GC code
   (replacing it with some per-thread data structures and different
   tricky GC code :)

 - wakeups are more prompt: parallel/concurrent performance should
   benefit.  I haven't seen anything dramatic in the parallel
   benchmarks so far, but a couple of threading benchmarks do improve
   a bit.

 - waking up a thread blocked on a blackhole is now O(1) (e.g. if
   it is the target of throwTo).

 - less sharing and better separation of Capabilities: communication
   is done with messages, the data structures are strictly owned by a
   Capability and cannot be modified except by sending messages.

 - this change will utlimately enable us to do more intelligent
   scheduling when threads block on each other.  This is what started
   off the whole thing, but it isn't done yet (#3838).

I'll be documenting all this on the wiki in due course.

1 files changed, 1 insertions, 1 deletions

diff --git a/rts/STM.c b/rts/STM.c
index be61538434..f98d201b07 100644
--- a/rts/STM.c
+++ b/rts/STM.c
@@ -377,7 +377,7 @@ static void unpark_tso(Capability *cap, StgTSO *tso) {
     lockTSO(tso);
     if (tso -> why_blocked == BlockedOnSTM) {
 	TRACE("unpark_tso on tso=%p", tso);
-	unblockOne(cap,tso);
+        tryWakeupThread(cap,tso);
     } else {
 	TRACE("spurious unpark_tso on tso=%p", tso);
     }