8341819: LightweightSynchronizer::enter_for races with deflation

Reviewed-by: pchilanomate, rkennke

src/hotspot/share/runtime/lightweightSynchronizer.cpp

@@ -635,8 +635,11 @@ void LightweightSynchronizer::enter_for(Handle obj, BasicLock* lock, JavaThread*
     bool entered = monitor->enter_for(locking_thread);
     assert(entered, "recursive ObjectMonitor::enter_for must succeed");
   } else {
-    // It is assumed that enter_for must enter on an object without contention.
-    monitor = inflate_and_enter(obj(), ObjectSynchronizer::inflate_cause_monitor_enter, locking_thread, current);
+    do {
+      // It is assumed that enter_for must enter on an object without contention.
+      monitor = inflate_and_enter(obj(), ObjectSynchronizer::inflate_cause_monitor_enter, locking_thread, current);
+      // But there may still be a race with deflation.
+    } while (monitor == nullptr);
   }
 
   assert(monitor != nullptr, "LightweightSynchronizer::enter_for must succeed");

test/jdk/com/sun/jdi/EATests.java

@@ -158,6 +158,19 @@
  *                 -XX:+DoEscapeAnalysis -XX:+EliminateAllocations -XX:+EliminateLocks -XX:+EliminateNestedLocks
  *                 -XX:LockingMode=0
  *                 -XX:GuaranteedAsyncDeflationInterval=1000
+ *
+ * @bug 8341819
+ * @comment Regression test for re-locking racing with deflation with LM_LIGHTWEIGHT.
+ * @run driver EATests
+ *                 -XX:+UnlockDiagnosticVMOptions
+ *                 -Xms256m -Xmx256m
+ *                 -Xbootclasspath/a:.
+ *                 -XX:CompileCommand=dontinline,*::dontinline_*
+ *                 -XX:+WhiteBoxAPI
+ *                 -Xbatch
+ *                 -XX:+DoEscapeAnalysis -XX:+EliminateAllocations -XX:+EliminateLocks -XX:+EliminateNestedLocks
+ *                 -XX:LockingMode=2
+ *                 -XX:GuaranteedAsyncDeflationInterval=1
  */
 
 /**