Is Happens-Before Consistency Guaranteed in Every Execution Order?

Happens-Before Consistency: An In-Depth Understanding

Question 1: Understanding the Definition

The given definition of happens-before consistency states that for a set of actions A, if a read action r in A sees a write action W(r), then it cannot be the case that hb(r, W(r)) or that there exists a write w in A such that w.v = r.v and hb(W(r), w) and hb(w, r). Your understanding that it equates to "..., it is the case that neither ... nor ..." is correct.

Question 2: Meaning of "w.v = r.v"

"w.v = r.v" indicates that the value of the read action r (represented by r.v) is equal to the value of the write action w (represented by w.v).

Question 3: Meaning of Left Numbers in Execution Orders

The left numbers in the execution orders represent the sequential order of actions within each thread. For example, in the first execution order, "1: B = 1" indicates that the first action in Thread 1 is the write to variable B.

Question 4: Visibility of Non-Volatile Variables

Your understanding that both r2 and r1 can see initial write values of 0 because A and B are not volatile fields is correct. Non-volatile variables can be stored in registers, potentially making them invisible to other threads for an extended period.

Question 5: Happens-Before Consistency in Second Execution Order

The second execution order is indeed happens-before consistent. Reads r2 and r1 occur after the writes to A and B, respectively, and there are no intervening actions that violate the happens-before relationship.

Question 6: Real-World Situations with Read-After-Write

In real-world applications, read-after-write situations can occur in various scenarios. For example, consider a multithreaded application where one thread modifies a shared object and another thread reads the same object without synchronization. Without appropriate mechanisms like volatile variables or synchronization, the reading thread may end up seeing an outdated value.

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