Using an atomic read-modify-write operation in a release sequence

Say, I want to create an object of type Foo in thread #1 and be able to access it in thread #3.
I can try something like:

std::atomic<int> sync{10};
Foo *fp;

// thread 1: modifies sync: 10 -> 11
fp = new Foo;
sync.store(11, std::memory_order_release);

// thread 2a: modifies sync: 11 -> 12
while (sync.load(std::memory_order_relaxed) != 11);
sync.store(12, std::memory_order_relaxed);

// thread 3
while (sync.load(std::memory_order_acquire) != 12);
fp->do_something();

• The store/release in thread #1 orders Foo with the update to 11
• thread #2a non-atomically increments the value of sync to 12
• the synchronizes-with relationship between thread #1 and #3 is only established when #3 loads 11

The scenario is broken because thread #3 spins until it loads 12, which may arrive out of order (wrt 11) and Foo is not ordered with 12 (due to the relaxed operations in thread #2a).
This is somewhat counter-intuitive since the modification order of sync is 10 → 11 → 12

The standard says (§ 1.10.1-6):

an atomic store-release synchronizes with a load-acquire that takes its value from the store (29.3). [ Note: Except in the specified cases, reading a later value does not necessarily ensure visibility as described below. Such a requirement would sometimes interfere with efficient implementation. —end note ]

It also says in (§ 1.10.1-5):

A release sequence headed by a release operation A on an atomic object M is a maximal contiguous subsequence of side effects in the modification order of M, where the first operation is A, and every subsequent operation
- is performed by the same thread that performed A, or
- is an atomic read-modify-write operation.

Now, thread #2a is modified to use an atomic read-modify-write operation:

// thread 2b: modifies sync: 11 -> 12
int val;
while ((val = 11) && !sync.compare_exchange_weak(val, 12, std::memory_order_relaxed));

If this release sequence is correct, Foo is synchronized with thread #3 when it loads either 11 or 12. My questions about the use of an atomic read-modify-write are:

• Does the scenario with thread #2b constitute a correct release sequence ?

And if so:

• Possibly platform specific, but is there a way to visualize the ordering of Foo with respect to sync updates 11 and 12 ?
• What are the specific properties of a read-modify-write operation that ensure this scenario is correct ?