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8261495: Shenandoah: reconsider update references memory ordering #2498

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@@ -141,7 +141,7 @@ inline void ShenandoahHeap::conc_update_with_forwarded(T* p) {
// logical object with its forwardee. The reason why we need stronger-than-relaxed memory
// ordering has to do with coordination with GC barriers and mutator accesses.
//
// In essence, stronger CAS access is required to maintain the transitive acq/rel that mutator
// In essence, stronger CAS access is required to maintain the transitive chains that mutator
// accesses build by themselves. To illustrate this point, consider the following example.
//
// Suppose "o" is the object that has a field "x" and the reference to "o" is stored
@@ -152,17 +152,17 @@ inline void ShenandoahHeap::conc_update_with_forwarded(T* p) {
// Thread 1 (Java)
// // --- previous access starts here
// ...
// T1.1: store(&o.x, 1, mo_relaxed);
// T1.2: store(&addr, o, mo_release); // volatile store
// T1.1: store(&o.x, 1, mo_relaxed)
// T1.2: store(&addr, o, mo_release) // volatile store
//
// // --- new access starts here
// // LRB: copy and install the new copy to fwdptr
// T1.3: var copy = copy(o)
// T1.4: cas(&fwd, t, copy, mo_release)
// T1.4: cas(&fwd, t, copy, mo_release) // pointer-mediated publication
// <access continues>
//
// Thread 2 (GC updater)
// T2.1: var f = load(&fwd, mo_acquire)
// T2.1: var f = load(&fwd, mo_{consume|acquire}) // pointer-mediated acquisition
// T2.2: cas(&addr, o, f, mo_release) // this method
//
// Thread 3 (Java)
@@ -179,7 +179,7 @@ inline void ShenandoahHeap::conc_update_with_forwarded(T* p) {
// from T2.2 to T3.1 (which is brought by this CAS).
//
// Note that we do not need to "acquire" in these methods, because we do not read the
// failure witnesses contents on any path.
// failure witnesses contents on any path, and "release" is enough.
//

inline void ShenandoahHeap::atomic_update_oop(oop update, oop* addr, oop compare) {