clarify situation of non-synchronized differently-sized accesses

Author: RalfJung

library/core/src/sync/atomic.rs

@@ -43,7 +43,8 @@
 //!
 //! That said, Rust *does* inherit the C++ limitation that non-synchronized atomic accesses may not
 //! partially overlap: they must be either disjoint or access the exact same memory. This in
-//! particular rules out non-synchronized differently-sized accesses to the same data.
+//! particular rules out non-synchronized differently-sized atomic accesses to the same data.
+//! (Non-synchronized differently-sized non-atomic read accesses are permitted.)
 //!
 //! [cpp]: https://en.cppreference.com/w/cpp/atomic
 //! [cpp-intro.races]: https://timsong-cpp.github.io/cppwp/n4868/intro.multithread#intro.races

src/tools/miri/src/concurrency/data_race.rs

@@ -304,8 +304,7 @@ impl AccessType {
     }
 }
 
-/// Memory Cell vector clock metadata
-/// for data-race detection.
+/// Per-byte vector clock metadata for data-race detection.
 #[derive(Clone, PartialEq, Eq, Debug)]
 struct MemoryCellClocks {
     /// The vector-clock timestamp and the thread that did the last non-atomic write. We don't need
@@ -324,8 +323,8 @@ struct MemoryCellClocks {
     read: VClock,
 
     /// Atomic access, acquire, release sequence tracking clocks.
-    /// For non-atomic memory in the common case this
-    /// value is set to None.
+    /// For non-atomic memory this value is set to None.
+    /// For atomic memory, each byte carries this information.
     atomic_ops: Option<Box<AtomicMemoryCellClocks>>,
 }
 

src/tools/miri/tests/fail/data_race/mixed_size_read.rs

@@ -10,7 +10,7 @@ fn convert(a: &AtomicU16) -> &[AtomicU8; 2] {
 }
 
 // We can't allow mixed-size accesses; they are not possible in C++ and even
-// Intel says you shouldn't do it.
+// Intel says you shouldn't do it. Even read-read races are disallowed that way.
 fn main() {
     let a = AtomicU16::new(0);
     let a16 = &a;