Rollup merge of rust-lang#44595 - budziq:stabilize_compiler_fences, r=alexcrichton

stabilized compiler_fences (fixes rust-lang#41091)

I did not know what to proceed with "unstable-book" entry. The feature would no longer be unstable so I have deleted it. If it was the wrong call I'll revert it (unfortunately his case is not described in the CONTRIBUTING.md).

Author: TimNN

src/doc/unstable-book/src/library-features/compiler-fences.md

@@ -1679,10 +1679,14 @@ pub fn fence(order: Ordering) {
 
 /// A compiler memory fence.
 ///
-/// `compiler_fence` does not emit any machine code, but prevents the compiler from re-ordering
-/// memory operations across this point. Which reorderings are disallowed is dictated by the given
-/// [`Ordering`]. Note that `compiler_fence` does *not* introduce inter-thread memory
-/// synchronization; for that, a [`fence`] is needed.
+/// `compiler_fence` does not emit any machine code, but restricts the kinds
+/// of memory re-ordering the compiler is allowed to do. Specifically, depending on
+/// the given [`Ordering`] semantics, the compiler may be disallowed from moving reads
+/// or writes from before or after the call to the other side of the call to
+/// `compiler_fence`. Note that it does **not** prevent the *hardware*
+/// from doing such re-ordering. This is not a problem in a single-threaded,
+/// execution context, but when other threads may modify memory at the same
+/// time, stronger synchronization primitives such as [`fence`] are required.
 ///
 /// The re-ordering prevented by the different ordering semantics are:
 ///
@@ -1691,19 +1695,64 @@ pub fn fence(order: Ordering) {
 ///  - with [`Acquire`], subsequent reads and writes cannot be moved ahead of preceding reads.
 ///  - with [`AcqRel`], both of the above rules are enforced.
 ///
+/// `compiler_fence` is generally only useful for preventing a thread from
+/// racing *with itself*. That is, if a given thread is executing one piece
+/// of code, and is then interrupted, and starts executing code elsewhere
+/// (while still in the same thread, and conceptually still on the same
+/// core). In traditional programs, this can only occur when a signal
+/// handler is registered. In more low-level code, such situations can also
+/// arise when handling interrupts, when implementing green threads with
+/// pre-emption, etc. Curious readers are encouraged to read the Linux kernel's
+/// discussion of [memory barriers].
+///
 /// # Panics
 ///
 /// Panics if `order` is [`Relaxed`].
 ///
+/// # Examples
+///
+/// Without `compiler_fence`, the `assert_eq!` in following code
+/// is *not* guaranteed to succeed, despite everything happening in a single thread.
+/// To see why, remember that the compiler is free to swap the stores to
+/// `IMPORTANT_VARIABLE` and `IS_READ` since they are both
+/// `Ordering::Relaxed`. If it does, and the signal handler is invoked right
+/// after `IS_READY` is updated, then the signal handler will see
+/// `IS_READY=1`, but `IMPORTANT_VARIABLE=0`.
+/// Using a `compiler_fence` remedies this situation.
+///
+/// ```
+/// use std::sync::atomic::{AtomicBool, AtomicUsize};
+/// use std::sync::atomic::{ATOMIC_BOOL_INIT, ATOMIC_USIZE_INIT};
+/// use std::sync::atomic::Ordering;
+/// use std::sync::atomic::compiler_fence;
+///
+/// static IMPORTANT_VARIABLE: AtomicUsize = ATOMIC_USIZE_INIT;
+/// static IS_READY: AtomicBool = ATOMIC_BOOL_INIT;
+///
+/// fn main() {
+///     IMPORTANT_VARIABLE.store(42, Ordering::Relaxed);
+///     // prevent earlier writes from being moved beyond this point
+///     compiler_fence(Ordering::Release);
+///     IS_READY.store(true, Ordering::Relaxed);
+/// }
+///
+/// fn signal_handler() {
+///     if IS_READY.load(Ordering::Relaxed) {
+///         assert_eq!(IMPORTANT_VARIABLE.load(Ordering::Relaxed), 42);
+///     }
+/// }
+/// ```
+///
 /// [`fence`]: fn.fence.html
 /// [`Ordering`]: enum.Ordering.html
 /// [`Acquire`]: enum.Ordering.html#variant.Acquire
 /// [`SeqCst`]: enum.Ordering.html#variant.SeqCst
 /// [`Release`]: enum.Ordering.html#variant.Release
 /// [`AcqRel`]: enum.Ordering.html#variant.AcqRel
 /// [`Relaxed`]: enum.Ordering.html#variant.Relaxed
+/// [memory barriers]: https://www.kernel.org/doc/Documentation/memory-barriers.txt
 #[inline]
-#[unstable(feature = "compiler_fences", issue = "41091")]
+#[stable(feature = "compiler_fences", since = "1.22.0")]
 pub fn compiler_fence(order: Ordering) {
     unsafe {
         match order {