Fine-tune the ordering for received

[wang384670111]

crossbeam/crossbeam-channel/src/flavors/at.rs

Line 51 in 95d0bd0

if !self.received.swap(true, Ordering::SeqCst) {

crossbeam/crossbeam-channel/src/flavors/at.rs

Line 89 in 95d0bd0

if !self.received.swap(true, Ordering::SeqCst) {

crossbeam/crossbeam-channel/src/flavors/at.rs

Line 120 in 95d0bd0

self.received.load(Ordering::SeqCst)

I believe that using a Release operation in swap and an Acquire operation in load can effectively synchronize with the writing of the message to memory. Using SeqCst is not necessary in this context.

(glad to make a pull request if this seems sensible)

[wang384670111]

Similarly, in wake.rs, using SeqCst is unnecessary. Using Acquire/Release would be sufficient.

crossbeam/crossbeam-channel/src/waker.rs

Lines 200 to 203 in 95d0bd0

	self.is_empty.store(
	inner.selectors.is_empty() && inner.observers.is_empty(),
	Ordering::SeqCst,
	);

crossbeam/crossbeam-channel/src/waker.rs

Lines 211 to 214 in 95d0bd0

	self.is_empty.store(
	inner.selectors.is_empty() && inner.observers.is_empty(),
	Ordering::SeqCst,
	);

crossbeam/crossbeam-channel/src/waker.rs

Line 221 in 95d0bd0

if !self.is_empty.load(Ordering::SeqCst) {

crossbeam/crossbeam-channel/src/waker.rs

Line 223 in 95d0bd0

if !self.is_empty.load(Ordering::SeqCst) {

crossbeam/crossbeam-channel/src/waker.rs

Lines 226 to 229 in 95d0bd0

	self.is_empty.store(
	inner.selectors.is_empty() && inner.observers.is_empty(),
	Ordering::SeqCst,
	);

crossbeam/crossbeam-channel/src/waker.rs

Lines 239 to 242 in 95d0bd0

	self.is_empty.store(
	inner.selectors.is_empty() && inner.observers.is_empty(),
	Ordering::SeqCst,
	);

crossbeam/crossbeam-channel/src/waker.rs

Lines 250 to 253 in 95d0bd0

	self.is_empty.store(
	inner.selectors.is_empty() && inner.observers.is_empty(),
	Ordering::SeqCst,
	);

crossbeam/crossbeam-channel/src/waker.rs

Lines 261 to 264 in 95d0bd0

	self.is_empty.store(
	inner.selectors.is_empty() && inner.observers.is_empty(),
	Ordering::SeqCst,
	);

[ibraheemdev]

The SeqCst throughout crossbeam-channel is necessary to establish a total order between send/recv and is_empty. Without it the optimistic check in https://github.com/crossbeam-rs/crossbeam/blob/master/crossbeam-channel/src/waker.rs#L222 would be incoherent.