diff --git a/lib/fluent/plugin/buffer.rb b/lib/fluent/plugin/buffer.rb
index be66f3e87e..954d07c7c2 100644
--- a/lib/fluent/plugin/buffer.rb
+++ b/lib/fluent/plugin/buffer.rb
@@ -313,22 +313,24 @@ def queued?(metadata=nil)
 
       def enqueue_chunk(metadata)
         log.trace "enqueueing chunk", instance: self.object_id, metadata: metadata
-        synchronize do
-          chunk = @stage.delete(metadata)
-          return nil unless chunk
+        chunk = synchronize do
+          @stage.delete(metadata)
+        end
+        return nil unless chunk
 
-          chunk.synchronize do
+        chunk.synchronize do
+          synchronize do
             if chunk.empty?
               chunk.close
             else
               @queue << chunk
               @queued_num[metadata] = @queued_num.fetch(metadata, 0) + 1
-              chunk.enqueued! if chunk.respond_to?(:enqueued!)
+              chunk.enqueued!
             end
+            bytesize = chunk.bytesize
+            @stage_size -= bytesize
+            @queue_size += bytesize
           end
-          bytesize = chunk.bytesize
-          @stage_size -= bytesize
-          @queue_size += bytesize
         end
         nil
       end
@@ -340,7 +342,7 @@ def enqueue_unstaged_chunk(chunk)
             metadata = chunk.metadata
             @queue << chunk
             @queued_num[metadata] = @queued_num.fetch(metadata, 0) + 1
-            chunk.enqueued! if chunk.respond_to?(:enqueued!)
+            chunk.enqueued!
           end
           @queue_size += chunk.bytesize
         end
@@ -348,17 +350,18 @@ def enqueue_unstaged_chunk(chunk)
 
       def enqueue_all
         log.trace "enqueueing all chunks in buffer", instance: self.object_id
-        synchronize do
-          if block_given?
-            @stage.keys.each do |metadata|
-              chunk = @stage[metadata]
-              v = yield metadata, chunk
-              enqueue_chunk(metadata) if v
-            end
-          else
-            @stage.keys.each do |metadata|
-              enqueue_chunk(metadata)
-            end
+        if block_given?
+          synchronize{ @stage.keys }.each do |metadata|
+            # NOTE: The following line might cause data race depending on Ruby implementations except CRuby
+            # cf. https://github.com/fluent/fluentd/pull/1721#discussion_r146170251
+            chunk = @stage[metadata]
+            next unless chunk
+            v = yield metadata, chunk
+            enqueue_chunk(metadata) if v
+          end
+        else
+          synchronize{ @stage.keys }.each do |metadata|
+            enqueue_chunk(metadata)
           end
         end
       end