Fix race condition in wakeupThreadOnCapability() (#2574)

wakeupThreadOnCapbility() is used to signal another capability that
there is a thread waiting to be added to its run queue.  It adds the
thread to the (locked) wakeup queue on the remote capability.  In
order to do this, it has to modify the TSO's link field, which has a
write barrier.  The write barrier might put the TSO on the mutable
list, and the bug was that it was using the mutable list of the
*target* capability, which we do not have exclusive access to.  We
should be using the current Capabilty's mutable list in this case.

1 files changed, 10 insertions, 5 deletions

diff --git a/rts/Schedule.h b/rts/Schedule.h
index 59bdb9e999..45aa000758 100644
--- a/rts/Schedule.h
+++ b/rts/Schedule.h
@@ -237,16 +237,21 @@ appendToBlockedQueue(StgTSO *tso)
 #endif
 
 #if defined(THREADED_RTS)
+// Assumes: my_cap is owned by the current Task.  We hold
+// other_cap->lock, but we do not necessarily own other_cap; another
+// Task may be running on it.
 INLINE_HEADER void
-appendToWakeupQueue (Capability *cap, StgTSO *tso)
+appendToWakeupQueue (Capability *my_cap, Capability *other_cap, StgTSO *tso)
 {
     ASSERT(tso->_link == END_TSO_QUEUE);
-    if (cap->wakeup_queue_hd == END_TSO_QUEUE) {
-	cap->wakeup_queue_hd = tso;
+    if (other_cap->wakeup_queue_hd == END_TSO_QUEUE) {
+	other_cap->wakeup_queue_hd = tso;
     } else {
-	setTSOLink(cap, cap->wakeup_queue_tl, tso);
+        // my_cap is passed to setTSOLink() because it may need to
+        // write to the mutable list.
+	setTSOLink(my_cap, other_cap->wakeup_queue_tl, tso);
     }
-    cap->wakeup_queue_tl = tso;
+    other_cap->wakeup_queue_tl = tso;
 }
 #endif