Replace non-atomic load with an atomic one

.github/workflows/ci.yml

@@ -15,3 +15,13 @@ jobs:
       - run: cargo fmt -- --check
       - run: cargo test
       - run: cargo bench
+  miri:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install Rust
+        run: rustup toolchain install nightly --component miri && rustup default nightly
+      - run: cargo miri test
+        env:
+          MIRIFLAGS: -Zmiri-strict-provenance -Zmiri-symbolic-alignment-check -Zmiri-disable-isolation
+          RUSTFLAGS: ${{ env.RUSTFLAGS }} -Z randomize-layout

src/lib.rs

@@ -216,8 +216,7 @@ impl<T: Send> ThreadLocal<T> {
         }
         unsafe {
             let entry = &*bucket_ptr.add(thread.index);
-            // Read without atomic operations as only this thread can set the value.
-            if (&entry.present as *const _ as *const bool).read() {
+            if entry.present.load(Ordering::Relaxed) {
                 Some(&*(&*entry.value.get()).as_ptr())
             } else {
                 None
@@ -610,6 +609,28 @@ mod tests {
         assert_eq!(vec![1, 2, 3], v);
     }
 
+    #[test]
+    fn miri_iter_soundness_check() {
+        let tls = Arc::new(ThreadLocal::new());
+        let _local = tls.get_or(|| Box::new(1));
+
+        let tls2 = tls.clone();
+        let join_1 = thread::spawn(move || {
+            let _tls = tls2.get_or(|| Box::new(2));
+            let iter = tls2.iter();
+            for item in iter {
+                println!("{:?}", item);
+            }
+        });
+
+        let iter = tls.iter();
+        for item in iter {
+            println!("{:?}", item);
+        }
+
+        join_1.join().ok();
+    }
+
     #[test]
     fn test_drop() {
         let local = ThreadLocal::new();