Fix data race in SamplingCircularQueue

src/circular-queue.h

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 27 matching lines @@
 // previous unread records are overwritten. The queue is designed with
 // a goal in mind to evade cache lines thrashing by preventing
 // simultaneous reads and writes to adjanced memory locations.
 //
 // IMPORTANT: as a producer never checks for chunks cleanness, it is
 // possible that it can catch up and overwrite a chunk that a consumer
 // is currently reading, resulting in a corrupt record being read.
 class SamplingCircularQueue {
  public:
   // Executed on the application thread.
-  SamplingCircularQueue(int record_size_in_bytes,
-                        int desired_chunk_size_in_bytes,
+  SamplingCircularQueue(size_t record_size_in_bytes,
+                        size_t desired_chunk_size_in_bytes,
                         int buffer_size_in_chunks);
   ~SamplingCircularQueue();
 
   // Enqueue returns a pointer to a memory location for storing the next
   // record.
   INLINE(void* Enqueue());
 
   // Executed on the consumer (analyzer) thread.
   // StartDequeue returns a pointer to a memory location for retrieving
   // the next record. After the record had been read by a consumer,
   // FinishDequeue must be called. Until that moment, subsequent calls
   // to StartDequeue will return the same pointer.
   void* StartDequeue();
   void FinishDequeue();
   // Due to a presence of slipping between the producer and the consumer,
   // the queue must be notified whether producing has been finished in order
   // to process remaining records from the buffer.
   void FlushResidualRecords();
 
   typedef AtomicWord Cell;
-  // Reserved values for the first cell of a record.
-  static const Cell kClear = 0;  // Marks clean (processed) chunks.
-  static const Cell kEnd = -1;   // Marks the end of the buffer.
 
  private:
+  // Reserved values for the chunk marker (first Cell in each chunk).
+  enum {
+    kClear,          // Marks clean (processed) chunks.
+    kEnqueueStarted  // Marks chunks where enqueue started.
+  };
+
   struct ProducerPosition {
+    Cell* next_chunk_pos;
     Cell* enqueue_pos;
   };
   struct ConsumerPosition {
     Cell* dequeue_chunk_pos;
     Cell* dequeue_chunk_poll_pos;
     Cell* dequeue_pos;
     Cell* dequeue_end_pos;
   };
 
   INLINE(void WrapPositionIfNeeded(Cell** pos));
 
   const int record_size_;
-  const int chunk_size_in_bytes_;
+  const size_t chunk_size_in_bytes_;
   const int chunk_size_;
   const int buffer_size_;
-  const int producer_consumer_distance_;
   Cell* buffer_;
   byte* positions_;
   ProducerPosition* producer_pos_;
   ConsumerPosition* consumer_pos_;
 
   DISALLOW_COPY_AND_ASSIGN(SamplingCircularQueue);
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_CIRCULAR_QUEUE_H_