RFR(T): 8246359: clarify confusing comment in ObjectMonitor::EnterI()'s race with async deflation

[Daniel D. Daugherty]

Greetings,

I have a trivial update to a comment that was added by the following fix
that I pushed late yesterday:

     JDK-8153224 Monitor deflation prolong safepoints
     https://bugs.openjdk.java.net/browse/JDK-8153224

Here's the tracking bug:

     JDK-8246359 clarify confusing comment in ObjectMonitor::EnterI()'s
                 race with async deflation
     https://bugs.openjdk.java.net/browse/JDK-8153224

The comment update is to resolve a 8153224 code review comment from
Carsten. He's already reviewed the comment on the 8153224 review thread
and is happy with the new comment.


And here's the context diffs for the fix:

$ hg diff
diff -r 629b14c63b75 src/hotspot/share/runtime/objectMonitor.cpp
--- a/src/hotspot/share/runtime/objectMonitor.cpp    Mon Jun 01 23:37:14 
2020 -0400
+++ b/src/hotspot/share/runtime/objectMonitor.cpp    Tue Jun 02 13:54:32 
2020 -0400
@@ -526,12 +526,17 @@

    if (AsyncDeflateIdleMonitors &&
        try_set_owner_from(DEFLATER_MARKER, Self) == DEFLATER_MARKER) {
-    // Cancelled the in-progress async deflation. We bump contentions an
-    // extra time to prevent the async deflater thread from temporarily
-    // changing it to -max_jint and back to zero (no flicker to confuse
-    // is_being_async_deflated()). The async deflater thread will
-    // decrement contentions after it recognizes that the async
-    // deflation was cancelled.
+    // Cancelled the in-progress async deflation by changing owner from
+    // DEFLATER_MARKER to Self. As part of the contended enter protocol,
+    // contentions was incremented to a positive value before EnterI()
+    // was called and that prevents the deflater thread from winning the
+    // last part of the 2-part async deflation protocol. After EnterI()
+    // returns to enter(), contentions is decremented because the caller
+    // now owns the monitor. We bump contentions an extra time here to
+    // prevent the deflater thread from winning the last part of the
+    // 2-part async deflation protocol after the regular decrement
+    // occurs in enter(). The deflater thread will decrement contentions
+    // after it recognizes that the async deflation was cancelled.
      add_to_contentions(1);
      assert(_succ != Self, "invariant");
      assert(_Responsible != Self, "invariant");
@@ -656,12 +661,17 @@

      if (AsyncDeflateIdleMonitors &&
          try_set_owner_from(DEFLATER_MARKER, Self) == DEFLATER_MARKER) {
-      // Cancelled the in-progress async deflation. We bump contentions an
-      // extra time to prevent the async deflater thread from temporarily
-      // changing it to -max_jint and back to zero (no flicker to confuse
-      // is_being_async_deflated()). The async deflater thread will
-      // decrement contentions after it recognizes that the async
-      // deflation was cancelled.
+      // Cancelled the in-progress async deflation by changing owner from
+      // DEFLATER_MARKER to Self. As part of the contended enter protocol,
+      // contentions was incremented to a positive value before EnterI()
+      // was called and that prevents the deflater thread from winning the
+      // last part of the 2-part async deflation protocol. After EnterI()
+      // returns to enter(), contentions is decremented because the caller
+      // now owns the monitor. We bump contentions an extra time here to
+      // prevent the deflater thread from winning the last part of the
+      // 2-part async deflation protocol after the regular decrement
+      // occurs in enter(). The deflater thread will decrement contentions
+      // after it recognizes that the async deflation was cancelled.
        add_to_contentions(1);
        break;
      }

Thanks, in advance, for any comments, questions or suggestions.

Dan