I'm the person that asked the original StackOverflow question.
After seeing the answers on StackOverflow and the discussions in the
mailing list, I still have several questions.

Question 1:
Does the "eligible to be unblocked" property/attribute have "time"
constraint?
Being "eligible to be unblocked by a particular atomic notifying
operation" is a property of an atomic waiting operation.
The wording in the standard text:
> A call to an atomic waiting operation on an atomic object M is
eligible to be unblocked by a call to an atomic notifying operation on M
if there exist side effects X and Y on M such that:
> (4.1) — the atomic waiting operation has blocked after observing the
result of X,
> (4.2) — X precedes Y in the modification order of M, and
> (4.3) — Y happens before the call to the atomic notifying operation.
What does the (4.1) clause mean? I see two different interpretations:
- A: "the atomic waiting operation IS blockING after observing the
result of X". i.e. this property holds only in a particular time span
(after it enters blocking state, before it exits blocking state).
- B: "the atomic waiting operation has ONCE EVER blocked after observing
the result of X". i.e. it is a fixed property of the atomic waiting
operation; if it is, it always is.
Which one is the correct interpretation, A or B?

Question 2:
Does the selection of atomic waiting operation to unblock have "time"
constraint?
The wording in the standard text:
> void notify_one() volatile noexcept;
> void notify_one() noexcept;
> Effects: Unblocks the execution of at least one atomic waiting
operation that is eligible to be unblocked (31.6) by this call, if any
such atomic waiting operations exist.
the phrase "if any such atomic waiting operations exist" also has two
possible interpretations:
- P: "if any such atomic waiting operations exist AT THE TIME of
execution of notify_one()"
- Q: "if any such atomic waiting operations EVER exist"
Which one is the correct interpretation, P or Q?

Question 3: What does an atomic notifying operation unblocks?
- X: it unblocks the whole atomic waiting operation. Once the (selected)
atomic waiting operation is unblocked, it is always unblocked.
- Y: it unblocks only one round of blocking of the atomic waiting
operation. If the atomic waiting operation wakes up and blocks again, it
needs another atomic notifying operation to unblock.
Which one is correct, X or Y?