some questions in ibgda

Hi, I have some questions about the following code in ibgda

```cpp
void ibgda_submit_requests(nvshmemi_ibgda_device_qp_t *qp, uint64_t base_wqe_idx,
uint32_t num_wqes, int message_idx = 0) {
nvshmemi_ibgda_device_qp_management_t *mvars = &qp->mvars;
uint64_t new_wqe_idx = base_wqe_idx + num_wqes;

// WQE writes must be finished first
__threadfence(); // (1)

// Wait for prior WQE slots to be filled first
auto *ready_idx = reinterpret_cast(&mvars->tx_wq.ready_head);
while (atomicCAS(ready_idx, base_wqe_idx, new_wqe_idx) != base_wqe_idx); // (2)

// Always post, not in batch
constexpr int kNumRequestInBatch = 4;
if (kAlwaysDoPostSend or (message_idx + 1) % kNumRequestInBatch == 0)
ibgda_post_send(qp, new_wqe_idx);
}
```

(1) I personally understand that the purpose of threadfence here is to ensure that writing to the WQE and writing to the DB do not occur out of order. From the view of the NIC, should threadfence_system be used instead?
(2) I personally understand that all threads executing atomicCAS have different compare/swap values, so is it necessary to use "atomic" operations in this case?

Thanks