Xcode中关于iOS中fenceExemptQueue的奇怪警告

当我的应用程序进入后台(之后applicationDidEnterBackground)时,会出现以下警告.

2015-12-11 20:54:43.661 AppName[759:124891]  *|synchronize-skip|* a fence was started inside of a snapshot block - skipping the workspace synchronize because it may dequeue messages from the fenceExemptQueue and snapshots expect that not to happen
2015-12-11 20:54:44.084 AppName[759:124891]  *|synchronize-skip|* a fence was started inside of a snapshot block - skipping the workspace synchronize because it may dequeue messages from the fenceExemptQueue and snapshots expect that not to happen

我在谷歌上找不到任何关于它的信息,也不知道为什么会这样.

有谁知道这意味着什么？谢谢!