book/resources: rm #[task_local] mention; add #[lock_free] example

the #[task_local] attribute was removed

book/en/src/by-example/resources.md

@@ -97,11 +97,15 @@ $ cargo run --example only-shared-access
 
 ## Lock-free resource access of mutable resources
 
-There exists two other options dealing with resources
+A critical section is *not* required to access a `#[shared]` resource that's only accessed by tasks running at the *same* priority.
+In this case, you can opt out of the `lock` API by adding the `#[lock_free]` field-level attribute to the resource declaration (see example below).
+Note that this is merely a convenience: if you do use the `lock` API, at runtime the framework will *not* produce a critical section.
 
-* `#[lock_free]`: there might be several tasks with the same priority
-  accessing the resource without critical section. Since tasks with the
-  same priority never can preempt another task on the same priority
-  this is safe.
-* `#[task_local]`: there must be only one task using this resource,
-  similar to a `static mut` task local resource, but (optionally) set-up by init.
+``` rust
+{{#include ../../../../examples/lock-free.rs}}
+```
+
+``` console
+$ cargo run --example lock-free
+{{#include ../../../../ci/expected/lock-free.run}}
+```

ci/expected/lock-free.run

@@ -0,0 +1,14 @@
+GPIOA/start
+  GPIOA/counter = 1
+GPIOA/end
+GPIOB/start
+  GPIOB/counter = 2
+GPIOB/end
+GPIOA/start
+  GPIOA/counter = 3
+GPIOA/end
+GPIOB/start
+  GPIOB/counter = 4
+GPIOB/end
+GPIOA/start
+  GPIOA/counter = 5

examples/lock-free.rs

@@ -0,0 +1,60 @@
+//! examples/lock-free.rs
+
+#![deny(unsafe_code)]
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+use panic_semihosting as _;
+
+#[rtic::app(device = lm3s6965)]
+mod app {
+    use cortex_m_semihosting::{debug, hprintln};
+    use lm3s6965::Interrupt;
+
+    #[shared]
+    struct Shared {
+        #[lock_free] // <- lock-free shared resource
+        counter: u64,
+    }
+
+    #[local]
+    struct Local {}
+
+    #[init]
+    fn init(_: init::Context) -> (Shared, Local, init::Monotonics) {
+        rtic::pend(Interrupt::GPIOA);
+
+        (Shared { counter: 0 }, Local {}, init::Monotonics())
+    }
+
+    #[task(binds = GPIOA, shared = [counter])] // <- same priority
+    fn gpioa(c: gpioa::Context) {
+        hprintln!("GPIOA/start").unwrap();
+        rtic::pend(Interrupt::GPIOB);
+
+        *c.shared.counter += 1; // <- no lock API required
+        let counter = *c.shared.counter;
+        hprintln!("  GPIOA/counter = {}", counter).unwrap();
+
+        if counter == 5 {
+            debug::exit(debug::EXIT_SUCCESS);
+        }
+        hprintln!("GPIOA/end").unwrap();
+    }
+
+    #[task(binds = GPIOB, shared = [counter])] // <- same priority
+    fn gpiob(c: gpiob::Context) {
+        hprintln!("GPIOB/start").unwrap();
+        rtic::pend(Interrupt::GPIOA);
+
+        *c.shared.counter += 1; // <- no lock API required
+        let counter = *c.shared.counter;
+        hprintln!("  GPIOB/counter = {}", counter).unwrap();
+
+        if counter == 5 {
+            debug::exit(debug::EXIT_SUCCESS);
+        }
+        hprintln!("GPIOB/end").unwrap();
+    }
+}