rts: Use relaxed operations for cap->running_task (TODO)

This shouldn't be necessary since only the owning thread of the capability
should be touching this.

2 files changed, 12 insertions, 10 deletions

diff --git a/rts/Capability.c b/rts/Capability.c
index 384d93c27e..0eaec444f8 100644
--- a/rts/Capability.c
+++ b/rts/Capability.c
@@ -525,7 +525,7 @@ releaseCapability_ (Capability* cap,
     ASSERT_RETURNING_TASKS(cap,task);
     ASSERT_LOCK_HELD(&cap->lock);
 
-    cap->running_task = NULL;
+    RELAXED_STORE(&cap->running_task, NULL);
 
     // Check to see whether a worker thread can be given
     // the go-ahead to return the result of an external call..
@@ -696,7 +696,7 @@ static Capability * waitForWorkerCapability (Task *task)
             cap->n_spare_workers--;
         }
 
-        cap->running_task = task;
+        RELAXED_STORE(&cap->running_task, task);
         RELEASE_LOCK(&cap->lock);
         break;
     }
@@ -737,7 +737,7 @@ static Capability * waitForReturnCapability (Task *task)
                 RELEASE_LOCK(&cap->lock);
                 continue;
             }
-            cap->running_task = task;
+            RELAXED_STORE(&cap->running_task, task);
             popReturningTask(cap);
             RELEASE_LOCK(&cap->lock);
             break;
@@ -763,8 +763,7 @@ static Capability * waitForReturnCapability (Task *task)
  * ------------------------------------------------------------------------- */
 static bool capability_is_busy(const Capability * cap)
 {
-    TSAN_ANNOTATE_BENIGN_RACE(&cap->running_task, "capability_is_busy");
-    return cap->running_task != NULL;
+    return RELAXED_LOAD(&cap->running_task) != NULL;
 }
 
 
@@ -798,7 +797,7 @@ static Capability * find_capability_for_task(const Task * task)
                   i += n_numa_nodes) {
                 // visits all the capabilities on this node, because
                 // cap[i]->node == i % n_numa_nodes
-                if (!capabilities[i]->running_task) {
+                if (!RELAXED_LOAD(&capabilities[i]->running_task)) {
                     return capabilities[i];
                 }
             }
@@ -848,7 +847,7 @@ void waitForCapability (Capability **pCap, Task *task)
     ACQUIRE_LOCK(&cap->lock);
     if (!cap->running_task) {
         // It's free; just grab it
-        cap->running_task = task;
+        RELAXED_STORE(&cap->running_task, task);
         RELEASE_LOCK(&cap->lock);
     } else {
         newReturningTask(cap,task);
@@ -1038,7 +1037,10 @@ bool
 tryGrabCapability (Capability *cap, Task *task)
 {
     int r;
-    if (cap->running_task != NULL) return false;
+    // N.B. This is benign as we will check again after taking the lock.
+    TSAN_ANNOTATE_BENIGN_RACE(&cap->running_task, "tryGrabCapability (cap->running_task)");
+    if (RELAXED_LOAD(&cap->running_task) != NULL) return false;
+
     r = TRY_ACQUIRE_LOCK(&cap->lock);
     if (r != 0) return false;
     if (cap->running_task != NULL) {
@@ -1046,7 +1048,7 @@ tryGrabCapability (Capability *cap, Task *task)
         return false;
     }
     task->cap = cap;
-    cap->running_task = task;
+    RELAXED_STORE(&cap->running_task, task);
     RELEASE_LOCK(&cap->lock);
     return true;
 }
diff --git a/rts/Task.c b/rts/Task.c
index 11ba5f1581..5fc2331827 100644
--- a/rts/Task.c
+++ b/rts/Task.c
@@ -473,7 +473,7 @@ startWorkerTask (Capability *cap)
   // else get in, because the new worker Task has nowhere to go to
   // sleep so that it could be woken up again.
   ASSERT_LOCK_HELD(&cap->lock);
-  cap->running_task = task;
+  RELAXED_STORE(&cap->running_task, task);
 
   // Set the name of the worker thread to the original process name followed by
   // ":w", but only if we're on Linux where the program_invocation_short_name