Further clarificarion for atomic and UnsafeCell docs:

- UnsafeCell: mention the term "data race", and reference the data race definition
- atomic: failing RMWs are just reads, reorder and reword docs

library/core/src/cell.rs

@@ -1895,11 +1895,17 @@ impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
 /// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain
 /// aliasing `&mut`, not even with `UnsafeCell<T>`.
 ///
+/// `UnsafeCell` does nothing to avoid data races; they are still undefined behavior. If multiple
+/// threads have access to the same `UnsafeCell`, they must follow the usual rules of the
+/// [concurrent memory model]: conflicting non-synchronized accesses must be done via the APIs in
+/// [`core::sync::atomic`].
+///
 /// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer
 /// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer
 /// correctly.
 ///
 /// [`.get()`]: `UnsafeCell::get`
+/// [concurrent memory model]: ../sync/atomic/index.html#memory-model-for-atomic-accesses
 ///
 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
 ///
@@ -1922,10 +1928,6 @@ impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
 /// live memory and the compiler is allowed to insert spurious reads if it can prove that this
 /// memory has not yet been deallocated.
 ///
-/// - At all times, you must avoid data races. If multiple threads have access to
-/// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
-/// accesses (or use atomics).
-///
 /// To assist with proper design, the following scenarios are explicitly declared legal
 /// for single-threaded code:
 ///

library/core/src/sync/atomic.rs

@@ -33,12 +33,6 @@
 //! atomic load (via the operations provided in this module). A "modification of an atomic object"
 //! refers to an atomic store.
 //!
-//! The most important aspect of this model is that conflicting non-synchronized accesses are
-//! Undefined Behavior unless both accesses are atomic. Here, accesses are *conflicting* if they
-//! affect overlapping regions of memory and at least one of them is a write. They are
-//! *non-synchronized* if neither of them *happens-before* the other, according to the
-//! happens-before order of the memory model.
-//!
 //! The end result is *almost* equivalent to saying that creating a *shared reference* to one of the
 //! Rust atomic types corresponds to creating an `atomic_ref` in C++, with the `atomic_ref` being
 //! destroyed when the lifetime of the shared reference ends. The main difference is that Rust
@@ -47,20 +41,25 @@
 //! objects" and "non-atomic objects" (with `atomic_ref` temporarily converting a non-atomic object
 //! into an atomic object).
 //!
-//! That said, Rust *does* inherit the C++ limitation that non-synchronized conflicting 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 atomic accesses to the
-//! same data unless all accesses are reads.
+//! The most important aspect of this model is that *data races* are undefined behavior. A data race
+//! is defined as conflicting non-synchronized accesses where at least one of the accesses is
+//! non-atomic. Here, accesses are *conflicting* if they affect overlapping regions of memory and at
+//! least one of them is a write. They are *non-synchronized* if neither of them *happens-before*
+//! the other, according to the happens-before order of the memory model.
 //!
-//! [cpp]: https://en.cppreference.com/w/cpp/atomic
-//! [cpp-intro.races]: https://timsong-cpp.github.io/cppwp/n4868/intro.multithread#intro.races
+//! The other possible cause of undefined behavior in the memory model are mixed-size accesses: Rust
+//! inherits the C++ limitation that non-synchronized conflicting atomic accesses may not partially
+//! overlap. In other words, every pair of non-synchronized atomic accesses must be either disjoint,
+//! access the exact same memory (including using the same access size), or both be reads.
 //!
-//! Each method takes an [`Ordering`] which represents the strength of
-//! the memory barrier for that operation. These orderings behave the
-//! same as the corresponding [C++20 atomic orderings][1]. For more information see the [nomicon][2].
+//! Each atomic access takes an [`Ordering`] which defines how the operation interacts with the
+//! happens-before order. These orderings behave the same as the corresponding [C++20 atomic
+//! orderings][cpp_memory_order]. For more information, see the [nomicon].
 //!
-//! [1]: https://en.cppreference.com/w/cpp/atomic/memory_order
-//! [2]: ../../../nomicon/atomics.html
+//! [cpp]: https://en.cppreference.com/w/cpp/atomic
+//! [cpp-intro.races]: https://timsong-cpp.github.io/cppwp/n4868/intro.multithread#intro.races
+//! [cpp_memory_order]: https://en.cppreference.com/w/cpp/atomic/memory_order
+//! [nomicon]: ../../../nomicon/atomics.html
 //!
 //! ```rust,no_run undefined_behavior
 //! use std::sync::atomic::{AtomicU16, AtomicU8, Ordering};
@@ -157,7 +156,7 @@
 //!
 //! # Atomic accesses to read-only memory
 //!
-//! In general, *all* atomic accesses on read-only memory are Undefined Behavior. For instance, attempting
+//! In general, *all* atomic accesses on read-only memory are undefined behavior. For instance, attempting
 //! to do a `compare_exchange` that will definitely fail (making it conceptually a read-only
 //! operation) can still cause a segmentation fault if the underlying memory page is mapped read-only. Since
 //! atomic `load`s might be implemented using compare-exchange operations, even a `load` can fault
@@ -173,7 +172,7 @@
 //!
 //! As an exception from the general rule stated above, "sufficiently small" atomic loads with
 //! `Ordering::Relaxed` are implemented in a way that works on read-only memory, and are hence not
-//! Undefined Behavior. The exact size limit for what makes a load "sufficiently small" varies
+//! undefined behavior. The exact size limit for what makes a load "sufficiently small" varies
 //! depending on the target:
 //!
 //! | `target_arch` | Size limit |

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

@@ -197,6 +197,22 @@ fn mixed_size_read_read() {
     });
 }
 
+fn failing_rmw_is_read() {
+    let a = AtomicUsize::new(0);
+    thread::scope(|s| {
+        s.spawn(|| unsafe {
+            // Non-atomic read.
+            let _val = *(&a as *const AtomicUsize).cast::<usize>();
+        });
+
+        s.spawn(|| {
+            // RMW that will fail.
+            // This is not considered a write, so there is no data race here.
+            a.compare_exchange(1, 2, Ordering::SeqCst, Ordering::SeqCst).unwrap_err();
+        });
+    });
+}
+
 pub fn main() {
     test_fence_sync();
     test_multiple_reads();
@@ -206,4 +222,5 @@ pub fn main() {
     test_read_read_race1();
     test_read_read_race2();
     mixed_size_read_read();
+    failing_rmw_is_read();
 }