Merge #787

787: Do not provide AtomicCell when cfg(crossbeam_loom) is enabled r=taiki-e a=taiki-e

We cannot provide AtomicCell under `cfg(crossbeam_loom)` because loom's atomic types have a different in-memory representation than the underlying type. (tokio-rs/loom#253)

https://github.com/crossbeam-rs/crossbeam/blob/bd83e1f2328e90298fc8a2f535a25d94ece09152/crossbeam-utils/src/atomic/atomic_cell.rs#L35-L42

fyi `@jonhoo` 

Co-authored-by: Taiki Endo <te316e89@gmail.com>

crossbeam-epoch/src/lib.rs

@@ -78,19 +78,7 @@ mod primitive {
     pub(crate) mod sync {
         pub(crate) mod atomic {
             use core::sync::atomic::Ordering;
-            pub(crate) use loom::sync::atomic::AtomicUsize;
-            pub(crate) fn fence(ord: Ordering) {
-                if let Ordering::Acquire = ord {
-                } else {
-                    // FIXME: loom only supports acquire fences at the moment.
-                    // https://github.com/tokio-rs/loom/issues/117
-                    // let's at least not panic...
-                    // this may generate some false positives (`SeqCst` is stronger than `Acquire`
-                    // for example), and some false negatives (`Relaxed` is weaker than `Acquire`),
-                    // but it's the best we can do for the time being.
-                }
-                loom::sync::atomic::fence(Ordering::Acquire)
-            }
+            pub(crate) use loom::sync::atomic::{fence, AtomicUsize};
 
             // FIXME: loom does not support compiler_fence at the moment.
             // https://github.com/tokio-rs/loom/issues/117