Cancellation safety

[Nugine]

Disclaimer: I know nothing about the details of RDMA before.

I have some questions about the example at first glance.

async-rdma/examples/rpc.rs

Lines 44 to 57 in 2a55f32

	/// Server can't aware of rdma `read` or `write`, so we need sync with client
	/// before client `read` and after `write`.
	async fn sync_with_client(rdma: &Rdma) {
	let mut lmr_sync = rdma
	.alloc_local_mr(Layout::new::<Request>())
	.map_err(|err| println!("{}", &err))
	.unwrap();
	//write data to lmr
	unsafe { *(lmr_sync.as_mut_ptr() as *mut Request) = Request::Sync };
	rdma.send(&lmr_sync)
	.await
	.map_err(|err| println!("{}", &err))
	.unwrap();
	}

1. When will the local memory region be deallocated?
2. Does the unsafe statement have any safety requirements?
3. What if the future returned by sync_with_client is dropped (cancelled) before the request completes?

Related article: https://zhuanlan.zhihu.com/p/346219893

[GTwhy]

Hi, thanks for your thoughtful questions and article.

1. For now, the relationship between LocalMr and RawMemoryRegion(contains ibv_mr : the "real" RDMA memory region) is one-to-one, and they are connected through Arc . So the memory region drop flow is : lmr.drop() -> raw_mr.drop() -> ibv_dreg_mr(ibv_mr). It's simple but inefficient, so we will transform jemalloc to manage ibv_mr and reuse it's preapplication and retain strategy in the next release.

2. The operations in the unsafe statement here almost degenerate to operations on raw memory pointers in C. I haven't thought carefully about how to pack this part better. I saw in the article that you have some thoughts about this. Maybe you have good ideas.

3. Good question! That's depending on the stage of the request. If the future dropped before or after ibv_post_send(rdma api) done, there will only be some error messages. But the handling of this during the execution of ibv_post_send has not been taken into account. This question is similar to the one mentioned in your article.

I will deal with the above issues later. You are welcome to contribute if you are interested, Thank you!