Capabiliity: Properly fix data race on n_returning_tasks

There is a real data race but can be made safe by using proper atomic
(but relaxed) accesses.

1 files changed, 8 insertions, 2 deletions

diff --git a/rts/Capability.c b/rts/Capability.c
index eb52505656..c0a06f21c9 100644
--- a/rts/Capability.c
+++ b/rts/Capability.c
@@ -211,7 +211,10 @@ newReturningTask (Capability *cap, Task *task)
         cap->returning_tasks_hd = task;
     }
     cap->returning_tasks_tl = task;
-    cap->n_returning_tasks++;
+
+    // See Note [Data race in shouldYieldCapability] in Schedule.c.
+    RELAXED_ADD(&cap->n_returning_tasks, 1);
+
     ASSERT_RETURNING_TASKS(cap,task);
 }
 
@@ -227,7 +230,10 @@ popReturningTask (Capability *cap)
         cap->returning_tasks_tl = NULL;
     }
     task->next = NULL;
-    cap->n_returning_tasks--;
+
+    // See Note [Data race in shouldYieldCapability] in Schedule.c.
+    RELAXED_ADD(&cap->n_returning_tasks, -1);
+
     ASSERT_RETURNING_TASKS(cap,task);
     return task;
 }