diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListLoader.java b/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListLoader.java
index 5444e4a..95172b0 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListLoader.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListLoader.java
@@ -86,6 +86,7 @@
   private final ThreeWayMergeStrategy mergeStrategy;
   private final ExecutorService diffExecutor;
   private final long timeoutMillis;
+  private final Object lock;
 
 
   @Inject
@@ -97,6 +98,7 @@
     patchListCache = plc;
     mergeStrategy = MergeUtil.getMergeStrategy(cfg);
     diffExecutor = de;
+    lock = new Object();
     timeoutMillis =
         ConfigUtil.getTimeUnit(cfg, "cache", PatchListCacheImpl.FILE_NAME,
             "timeout", TimeUnit.MILLISECONDS.convert(5, TimeUnit.SECONDS),
@@ -200,7 +202,9 @@
     Future<FileHeader> result = diffExecutor.submit(new Callable<FileHeader>() {
       @Override
       public FileHeader call() throws IOException {
-        return diffFormatter.toFileHeader(diffEntry);
+        synchronized (lock) {
+          return diffFormatter.toFileHeader(diffEntry);
+        }
       }
     });
 
@@ -214,7 +218,9 @@
                       + " comparing " + diffEntry.getOldId().name()
                       + ".." + diffEntry.getNewId().name());
       result.cancel(true);
-      return toFileHeaderWithoutMyersDiff(diffFormatter, diffEntry);
+      synchronized (lock) {
+        return toFileHeaderWithoutMyersDiff(diffFormatter, diffEntry);
+      }
     } catch (ExecutionException e) {
       // If there was an error computing the result, carry it
       // up to the caller so the cache knows this key is invalid.