Add page about spin.

src/SUMMARY.md

@@ -273,7 +273,7 @@
   - [aarch64-paging](bare-metal/useful-crates/aarch64-paging.md)
   - [buddy_system_allocator](bare-metal/useful-crates/buddy_system_allocator.md)
   - [tinyvec](bare-metal/useful-crates/tinyvec.md)
-  - [spin and once_cell]()
+  - [spin](bare-metal/useful-crates/spin.md)
 - [Android]()
   - [vmbase]()
 - [Exercises](exercises/bare-metal/afternoon.md)

src/bare-metal/useful-crates/spin.md

@@ -0,0 +1,32 @@
+# `spin`
+
+`std::sync::Mutex` and the other synchronisation primitives from `std::sync` are not available in
+`core` or `alloc`. How can we manage synchronisation or interior mutability, such as for sharing
+state between different CPUs?
+
+The [`spin`][1] crate provides spinlock-based equivalents of many of these primitives.
+
+```rust,editable,compile_fail
+use spin::mutex::SpinMutex;
+
+static counter: SpinMutex<u32> = SpinMutex::new(0);
+
+fn main() {
+    println!("count: {}", counter.lock());
+    *counter.lock() += 2;
+    println!("count: {}", counter.lock());
+}
+```
+
+<details>
+
+* Be careful to avoid deadlock if you take locks in interrupt handlers.
+* `spin` also has a ticket lock mutex implementation; equivalents of `RwLock`, `Barrier` and `Once`
+  from `std::sync`;  and `Lazy` for lazy initialisation.
+* The [`once_cell`][2] crate also has some useful types for late initialisation with a slightly
+  different approach to `spin::once::Once`.
+
+</details>
+
+[1]: https://crates.io/crates/spin
+[2]: https://crates.io/crates/once_cell