(1) What does it really mean by saying “ordered before”? Because even if action_a happens-before action_b,action_a can be executed after action_b in some implementation,right?

發(fā)生之前是因果關(guān)系,而不是時(shí)間關(guān)系. action_a在action_b之前進(jìn)行因果排序,無論它是否在它之前實(shí)際執(zhí)行.然而,在實(shí)踐中,運(yùn)行時(shí)很難在沒有時(shí)間順序的情況下保持因果關(guān)系.查看my earlier question,其中詳細(xì)介紹了因果關(guān)系的主題.

(2) If action_a happens-before action_b,does it mean action_a MUST NOT see action_b? Or action_a may see or may not see action_b?

行動對另一方的可見性有一個明確的整體順序.這由指定格式良好的執(zhí)行的部分處理.因此,對于任何兩個動作a和b,a對b可見,或b對a可見,或者上述都不可見.理解精心構(gòu)造的執(zhí)行概念的良好讀物是Java Memory Model Examples: Good,Bad,and Ugly.

(3) If action_a does NOT happen-before action_b,and action_b does NOT happen-before action_a,does it mean action_a may see or may not see action_b?

是的,兩者都有可能.無論如何都無法保證.

(4) There could not be any cyclic happens-before,right?

Happens-before必須強(qiáng)加部分排序,而排序的關(guān)鍵屬性是無循環(huán).

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