code: void unpin(Buffer... buffs) {
  		for (Buffer buff : buffs) {
  			try {
  				// Get the lock of buffer
  				buff.getExternalLock().lock();
  				buff.unpin();
  				if (!buff.isPinned())
  					numAvailable.incrementAndGet();
  			} finally {
  				// Release the lock of buffer
  				buff.getExternalLock().unlock();
  			}
  		}
  	}
docs: Unpins the specified buffers.
  
  @param buffs
  the buffers to be unpinned