Added commentary

petermz · Aug 3, 2023 · 0e7a017 · 0e7a017
1 parent c1a0610
commit 0e7a017
Showing 1 changed file with 11 additions and 3 deletions.
diff --git a/.../src/com.oracle.svm.core/src/com/oracle/svm/core/monitor/MultiThreadedMonitorSupport.java b/.../src/com.oracle.svm.core/src/com/oracle/svm/core/monitor/MultiThreadedMonitorSupport.java
@@ -211,12 +211,16 @@ private static void slowPathMonitorEnter(Object obj) {
         try {
             int monitorOffset = getMonitorOffset(obj);
             if (monitorOffset == 0) {
-                /// slow path
+                // Slow path
                 singleton().monitorEnter(obj, MonitorInflationCause.MONITOR_ENTER);
                 return;
             }
 
-            /// optimized path
+            /*
+             * Optimized path takes advantage of the knowledge that, when a new monitor object is created,
+             * it is not shared with other threads, so we can set its state without CAS. It also has
+             * acquisitions == 1 by construction, so we don't need to set that too.
+             */
             long current = JavaThreads.getCurrentThreadId();
             JavaMonitor monitor = (JavaMonitor) BarrieredAccess.readObject(obj, monitorOffset);
             try {
@@ -295,11 +299,15 @@ private static void slowPathMonitorExit(Object obj) {
              */
             int monitorOffset = getMonitorOffset(obj);
             if (monitorOffset == 0) {
+                // Slow path
                 singleton().monitorExit(obj, MonitorInflationCause.VM_INTERNAL);
                 return;
             }
 
-            /// optimized path
+            /*
+             * Optimized path: we know that monitor object exists, due to structured locking,
+             * so does not need to be created/inflated.
+             */
             JavaMonitor monitor = (JavaMonitor) BarrieredAccess.readObject(obj, monitorOffset);
             if (monitor.acquisitions > 1) {
                 monitor.acquisitions--;