deadlock fix: reset the flag *after* checking the blackhole queue

diff --git a/rts/Schedule.c b/rts/Schedule.c
index 8ab964d..569c3d0 100644 (file)
--- a/rts/Schedule.c
+++ b/rts/Schedule.c
@@ -934,8 +934,13 @@ scheduleCheckBlackHoles (Capability *cap)
     {
        ACQUIRE_LOCK(&sched_mutex);
        if ( blackholes_need_checking ) {
-           checkBlackHoles(cap);
            blackholes_need_checking = rtsFalse;
+            // important that we reset the flag *before* checking the
+            // blackhole queue, otherwise we could get deadlock.  This
+            // happens as follows: we wake up a thread that
+            // immediately runs on another Capability, blocks on a
+            // blackhole, and then we reset the blackholes_need_checking flag.
+           checkBlackHoles(cap);
        }
        RELEASE_LOCK(&sched_mutex);
     }