HDFS-14751. Synchronize on diffs in DirectoryScanner. Contributed by Lisheng Sun.

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DirectoryScanner.java

@@ -349,7 +349,9 @@ public class DirectoryScanner implements Runnable {
    * Clear the current cache of diffs and statistics.
    */
   private void clear() {
+    synchronized (diffs) {
       diffs.clear();
+    }
     stats.clear();
   }
 
@@ -430,6 +432,7 @@ public class DirectoryScanner implements Runnable {
     // HDFS-14476: run checkAndUpadte with batch to avoid holding the lock too
     // long
     int loopCount = 0;
+    synchronized (diffs) {
       for (final Map.Entry<String, ScanInfo> entry : diffs.getEntries()) {
         dataset.checkAndUpdate(entry.getKey(), entry.getValue());
 
@@ -442,6 +445,7 @@ public class DirectoryScanner implements Runnable {
         }
         loopCount++;
       }
+    }
 
     if (!retainDiffs) {
       clear();
@@ -545,7 +549,9 @@ public class DirectoryScanner implements Runnable {
           }
           d++;
         }
+        synchronized (diffs) {
           diffs.addAll(bpid, diffRecord);
+        }
         LOG.info("Scan Results: {}", statsRecord);
       }
     }