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695 lines
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<h1 class="menu-title">Real-Time Interrupt-driven Concurrency</h1>
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<main>
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<h1 id="resource-usage"><a class="header" href="#resource-usage">Resource usage</a></h1>
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<p>The RTIC framework manages shared and task local resources allowing persistent data
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storage and safe accesses without the use of <code>unsafe</code> code.</p>
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<p>RTIC resources are visible only to functions declared within the <code>#[app]</code> module and the framework
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gives the user complete control (on a per-task basis) over resource accessibility.</p>
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<p>Declaration of system-wide resources is done by annotating <strong>two</strong> <code>struct</code>s within the <code>#[app]</code> module
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with the attribute <code>#[local]</code> and <code>#[shared]</code>.
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Each field in these structures corresponds to a different resource (identified by field name).
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The difference between these two sets of resources will be covered below.</p>
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<p>Each task must declare the resources it intends to access in its corresponding metadata attribute
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using the <code>local</code> and <code>shared</code> arguments. Each argument takes a list of resource identifiers.
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The listed resources are made available to the context under the <code>local</code> and <code>shared</code> fields of the
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<code>Context</code> structure.</p>
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<p>The <code>init</code> task returns the initial values for the system-wide (<code>#[shared]</code> and <code>#[local]</code>)
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resources, and the set of initialized timers used by the application. The monotonic timers will be
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further discussed in <a href="./monotonic.html">Monotonic & <code>spawn_{at/after}</code></a>.</p>
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<h2 id="local-resources"><a class="header" href="#local-resources"><code>#[local]</code> resources</a></h2>
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<p><code>#[local]</code> resources are locally accessible to a specific task, meaning that only that task can
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access the resource and does so without locks or critical sections. This allows for the resources,
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commonly drivers or large objects, to be initialized in <code>#[init]</code> and then be passed to a specific
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task.</p>
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<p>Thus, a task <code>#[local]</code> resource can only be accessed by one singular task.
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Attempting to assign the same <code>#[local]</code> resource to more than one task is a compile-time error.</p>
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<p>Types of <code>#[local]</code> resources must implement a <a href="https://doc.rust-lang.org/stable/core/marker/trait.Send.html"><code>Send</code></a> trait as they are being sent from <code>init</code>
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to a target task, crossing a thread boundary.</p>
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<p>The example application shown below contains two tasks where each task has access to its own
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<code>#[local]</code> resource; the <code>idle</code> task has its own <code>#[local]</code> as well.</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>//! examples/locals.rs
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#![deny(unsafe_code)]
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#![deny(warnings)]
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#![deny(missing_docs)]
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#![deny(missing_docs)]
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#![no_main]
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#![no_std]
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use panic_semihosting as _;
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#[rtic::app(device = lm3s6965, dispatchers = [UART0, UART1])]
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mod app {
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use cortex_m_semihosting::{debug, hprintln};
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#[shared]
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struct Shared {}
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#[local]
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struct Local {
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/// Local foo
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local_to_foo: i64,
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/// Local bar
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local_to_bar: i64,
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/// Local idle
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local_to_idle: i64,
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}
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// `#[init]` cannot access locals from the `#[local]` struct as they are initialized here.
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#[init]
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fn init(_: init::Context) -> (Shared, Local, init::Monotonics) {
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foo::spawn().unwrap();
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bar::spawn().unwrap();
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(
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Shared {},
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// initial values for the `#[local]` resources
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Local {
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local_to_foo: 0,
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local_to_bar: 0,
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local_to_idle: 0,
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},
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init::Monotonics(),
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)
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}
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// `local_to_idle` can only be accessed from this context
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#[idle(local = [local_to_idle])]
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fn idle(cx: idle::Context) -> ! {
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let local_to_idle = cx.local.local_to_idle;
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*local_to_idle += 1;
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hprintln!("idle: local_to_idle = {}", local_to_idle);
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debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
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// error: no `local_to_foo` field in `idle::LocalResources`
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// _cx.local.local_to_foo += 1;
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// error: no `local_to_bar` field in `idle::LocalResources`
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// _cx.local.local_to_bar += 1;
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loop {
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cortex_m::asm::nop();
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}
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}
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// `local_to_foo` can only be accessed from this context
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#[task(local = [local_to_foo])]
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fn foo(cx: foo::Context) {
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let local_to_foo = cx.local.local_to_foo;
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*local_to_foo += 1;
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// error: no `local_to_bar` field in `foo::LocalResources`
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// cx.local.local_to_bar += 1;
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hprintln!("foo: local_to_foo = {}", local_to_foo);
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}
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// `local_to_bar` can only be accessed from this context
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#[task(local = [local_to_bar])]
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fn bar(cx: bar::Context) {
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let local_to_bar = cx.local.local_to_bar;
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*local_to_bar += 1;
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// error: no `local_to_foo` field in `bar::LocalResources`
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// cx.local.local_to_foo += 1;
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hprintln!("bar: local_to_bar = {}", local_to_bar);
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}
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}
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<span class="boring">}</span></code></pre></pre>
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<p>Running the example:</p>
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<pre><code class="language-console">$ cargo run --target thumbv7m-none-eabi --example locals
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foo: local_to_foo = 1
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bar: local_to_bar = 1
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idle: local_to_idle = 1
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</code></pre>
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<p>Local resources in <code>#[init]</code> and <code>#[idle]</code> have <code>'static</code>
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lifetimes. This is safe since both tasks are not re-entrant.</p>
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<h3 id="task-local-initialized-resources"><a class="header" href="#task-local-initialized-resources">Task local initialized resources</a></h3>
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<p>Local resources can also be specified directly in the resource claim like so:
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<code>#[task(local = [my_var: TYPE = INITIAL_VALUE, ...])]</code>; this allows for creating locals which do no need to be
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initialized in <code>#[init]</code>.</p>
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<p>Types of <code>#[task(local = [..])]</code> resources have to be neither <a href="https://doc.rust-lang.org/stable/core/marker/trait.Send.html"><code>Send</code></a> nor <a href="https://doc.rust-lang.org/stable/core/marker/trait.Sync.html"><code>Sync</code></a> as they
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are not crossing any thread boundary.</p>
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<p>In the example below the different uses and lifetimes are shown:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>//! examples/declared_locals.rs
|
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|
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#![deny(unsafe_code)]
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#![deny(warnings)]
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#![deny(missing_docs)]
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#![no_main]
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|
#![no_std]
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|
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|
use panic_semihosting as _;
|
|
|
|
#[rtic::app(device = lm3s6965, dispatchers = [UART0])]
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mod app {
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use cortex_m_semihosting::debug;
|
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|
|
#[shared]
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struct Shared {}
|
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|
|
#[local]
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struct Local {}
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#[init(local = [a: u32 = 0])]
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fn init(cx: init::Context) -> (Shared, Local, init::Monotonics) {
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// Locals in `#[init]` have 'static lifetime
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let _a: &'static mut u32 = cx.local.a;
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debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
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(Shared {}, Local {}, init::Monotonics())
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}
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#[idle(local = [a: u32 = 0])]
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fn idle(cx: idle::Context) -> ! {
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|
// Locals in `#[idle]` have 'static lifetime
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|
let _a: &'static mut u32 = cx.local.a;
|
|
|
|
loop {}
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}
|
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|
|
#[task(local = [a: u32 = 0])]
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fn foo(cx: foo::Context) {
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|
// Locals in `#[task]`s have a local lifetime
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|
let _a: &mut u32 = cx.local.a;
|
|
|
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// error: explicit lifetime required in the type of `cx`
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// let _a: &'static mut u32 = cx.local.a;
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}
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}
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<span class="boring">}</span></code></pre></pre>
|
|
<!-- ``` console
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|
$ cargo run --target thumbv7m-none-eabi --example declared_locals
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|
|
``` -->
|
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<h2 id="shared-resources-and-lock"><a class="header" href="#shared-resources-and-lock"><code>#[shared]</code> resources and <code>lock</code></a></h2>
|
|
<p>Critical sections are required to access <code>#[shared]</code> resources in a data race-free manner and to
|
|
achieve this the <code>shared</code> field of the passed <code>Context</code> implements the <a href="../../../api/rtic/trait.Mutex.html"><code>Mutex</code></a> trait for each
|
|
shared resource accessible to the task. This trait has only one method, <a href="../../../api/rtic/trait.Mutex.html#method.lock"><code>lock</code></a>, which runs its
|
|
closure argument in a critical section.</p>
|
|
<p>The critical section created by the <code>lock</code> API is based on dynamic priorities: it temporarily
|
|
raises the dynamic priority of the context to a <em>ceiling</em> priority that prevents other tasks from
|
|
preempting the critical section. This synchronization protocol is known as the
|
|
<a href="https://en.wikipedia.org/wiki/Priority_ceiling_protocol">Immediate Ceiling Priority Protocol (ICPP)</a>, and complies with
|
|
<a href="https://en.wikipedia.org/wiki/Stack_Resource_Policy">Stack Resource Policy (SRP)</a> based scheduling of RTIC.</p>
|
|
<p>In the example below we have three interrupt handlers with priorities ranging from one to three.
|
|
The two handlers with the lower priorities contend for a <code>shared</code> resource and need to succeed in locking the
|
|
resource in order to access its data. The highest priority handler, which does not access the <code>shared</code>
|
|
resource, is free to preempt a critical section created by the lowest priority handler.</p>
|
|
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
|
|
</span><span class="boring">fn main() {
|
|
</span>//! examples/lock.rs
|
|
|
|
#![deny(unsafe_code)]
|
|
#![deny(warnings)]
|
|
#![deny(missing_docs)]
|
|
#![no_main]
|
|
#![no_std]
|
|
|
|
use panic_semihosting as _;
|
|
|
|
#[rtic::app(device = lm3s6965, dispatchers = [GPIOA, GPIOB, GPIOC])]
|
|
mod app {
|
|
use cortex_m_semihosting::{debug, hprintln};
|
|
|
|
#[shared]
|
|
struct Shared {
|
|
shared: u32,
|
|
}
|
|
|
|
#[local]
|
|
struct Local {}
|
|
|
|
#[init]
|
|
fn init(_: init::Context) -> (Shared, Local, init::Monotonics) {
|
|
foo::spawn().unwrap();
|
|
|
|
(Shared { shared: 0 }, Local {}, init::Monotonics())
|
|
}
|
|
|
|
// when omitted priority is assumed to be `1`
|
|
#[task(shared = [shared])]
|
|
fn foo(mut c: foo::Context) {
|
|
hprintln!("A");
|
|
|
|
// the lower priority task requires a critical section to access the data
|
|
c.shared.shared.lock(|shared| {
|
|
// data can only be modified within this critical section (closure)
|
|
*shared += 1;
|
|
|
|
// bar will *not* run right now due to the critical section
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|
bar::spawn().unwrap();
|
|
|
|
hprintln!("B - shared = {}", *shared);
|
|
|
|
// baz does not contend for `shared` so it's allowed to run now
|
|
baz::spawn().unwrap();
|
|
});
|
|
|
|
// critical section is over: bar can now start
|
|
|
|
hprintln!("E");
|
|
|
|
debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
|
|
}
|
|
|
|
#[task(priority = 2, shared = [shared])]
|
|
fn bar(mut c: bar::Context) {
|
|
// the higher priority task does still need a critical section
|
|
let shared = c.shared.shared.lock(|shared| {
|
|
*shared += 1;
|
|
|
|
*shared
|
|
});
|
|
|
|
hprintln!("D - shared = {}", shared);
|
|
}
|
|
|
|
#[task(priority = 3)]
|
|
fn baz(_: baz::Context) {
|
|
hprintln!("C");
|
|
}
|
|
}
|
|
<span class="boring">}</span></code></pre></pre>
|
|
<pre><code class="language-console">$ cargo run --target thumbv7m-none-eabi --example lock
|
|
A
|
|
B - shared = 1
|
|
C
|
|
D - shared = 2
|
|
E
|
|
</code></pre>
|
|
<p>Types of <code>#[shared]</code> resources have to be <a href="https://doc.rust-lang.org/stable/core/marker/trait.Send.html"><code>Send</code></a>.</p>
|
|
<h2 id="multi-lock"><a class="header" href="#multi-lock">Multi-lock</a></h2>
|
|
<p>As an extension to <code>lock</code>, and to reduce rightward drift, locks can be taken as tuples. The
|
|
following examples show this in use:</p>
|
|
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
|
|
</span><span class="boring">fn main() {
|
|
</span>//! examples/mutlilock.rs
|
|
|
|
#![deny(unsafe_code)]
|
|
#![deny(warnings)]
|
|
#![deny(missing_docs)]
|
|
#![no_main]
|
|
#![no_std]
|
|
|
|
use panic_semihosting as _;
|
|
|
|
#[rtic::app(device = lm3s6965, dispatchers = [GPIOA])]
|
|
mod app {
|
|
use cortex_m_semihosting::{debug, hprintln};
|
|
|
|
#[shared]
|
|
struct Shared {
|
|
shared1: u32,
|
|
shared2: u32,
|
|
shared3: u32,
|
|
}
|
|
|
|
#[local]
|
|
struct Local {}
|
|
|
|
#[init]
|
|
fn init(_: init::Context) -> (Shared, Local, init::Monotonics) {
|
|
locks::spawn().unwrap();
|
|
|
|
(
|
|
Shared {
|
|
shared1: 0,
|
|
shared2: 0,
|
|
shared3: 0,
|
|
},
|
|
Local {},
|
|
init::Monotonics(),
|
|
)
|
|
}
|
|
|
|
// when omitted priority is assumed to be `1`
|
|
#[task(shared = [shared1, shared2, shared3])]
|
|
fn locks(c: locks::Context) {
|
|
let s1 = c.shared.shared1;
|
|
let s2 = c.shared.shared2;
|
|
let s3 = c.shared.shared3;
|
|
|
|
(s1, s2, s3).lock(|s1, s2, s3| {
|
|
*s1 += 1;
|
|
*s2 += 1;
|
|
*s3 += 1;
|
|
|
|
hprintln!("Multiple locks, s1: {}, s2: {}, s3: {}", *s1, *s2, *s3);
|
|
});
|
|
|
|
debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
|
|
}
|
|
}
|
|
<span class="boring">}</span></code></pre></pre>
|
|
<pre><code class="language-console">$ cargo run --target thumbv7m-none-eabi --example multilock
|
|
Multiple locks, s1: 1, s2: 1, s3: 1
|
|
</code></pre>
|
|
<h2 id="only-shared---access"><a class="header" href="#only-shared---access">Only shared (<code>&-</code>) access</a></h2>
|
|
<p>By default, the framework assumes that all tasks require exclusive access (<code>&mut-</code>) to resources,
|
|
but it is possible to specify that a task only requires shared access (<code>&-</code>) to a resource using the
|
|
<code>&resource_name</code> syntax in the <code>shared</code> list.</p>
|
|
<p>The advantage of specifying shared access (<code>&-</code>) to a resource is that no locks are required to
|
|
access the resource even if the resource is contended by more than one task running at different
|
|
priorities. The downside is that the task only gets a shared reference (<code>&-</code>) to the resource,
|
|
limiting the operations it can perform on it, but where a shared reference is enough this approach
|
|
reduces the number of required locks. In addition to simple immutable data, this shared access can
|
|
be useful where the resource type safely implements interior mutability, with appropriate locking
|
|
or atomic operations of its own.</p>
|
|
<p>Note that in this release of RTIC it is not possible to request both exclusive access (<code>&mut-</code>)
|
|
and shared access (<code>&-</code>) to the <em>same</em> resource from different tasks. Attempting to do so will
|
|
result in a compile error.</p>
|
|
<p>In the example below a key (e.g. a cryptographic key) is loaded (or created) at runtime and then
|
|
used from two tasks that run at different priorities without any kind of lock.</p>
|
|
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
|
|
</span><span class="boring">fn main() {
|
|
</span>//! examples/only-shared-access.rs
|
|
|
|
#![deny(unsafe_code)]
|
|
#![deny(warnings)]
|
|
#![deny(missing_docs)]
|
|
#![no_main]
|
|
#![no_std]
|
|
|
|
use panic_semihosting as _;
|
|
|
|
#[rtic::app(device = lm3s6965, dispatchers = [UART0, UART1])]
|
|
mod app {
|
|
use cortex_m_semihosting::{debug, hprintln};
|
|
|
|
#[shared]
|
|
struct Shared {
|
|
key: u32,
|
|
}
|
|
|
|
#[local]
|
|
struct Local {}
|
|
|
|
#[init]
|
|
fn init(_: init::Context) -> (Shared, Local, init::Monotonics) {
|
|
foo::spawn().unwrap();
|
|
bar::spawn().unwrap();
|
|
|
|
(Shared { key: 0xdeadbeef }, Local {}, init::Monotonics())
|
|
}
|
|
|
|
#[task(shared = [&key])]
|
|
fn foo(cx: foo::Context) {
|
|
let key: &u32 = cx.shared.key;
|
|
hprintln!("foo(key = {:#x})", key);
|
|
|
|
debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
|
|
}
|
|
|
|
#[task(priority = 2, shared = [&key])]
|
|
fn bar(cx: bar::Context) {
|
|
hprintln!("bar(key = {:#x})", cx.shared.key);
|
|
}
|
|
}
|
|
<span class="boring">}</span></code></pre></pre>
|
|
<pre><code class="language-console">$ cargo run --target thumbv7m-none-eabi --example only-shared-access
|
|
bar(key = 0xdeadbeef)
|
|
foo(key = 0xdeadbeef)
|
|
</code></pre>
|
|
<h2 id="lock-free-resource-access-of-shared-resources"><a class="header" href="#lock-free-resource-access-of-shared-resources">Lock-free resource access of shared resources</a></h2>
|
|
<p>A critical section is <em>not</em> required to access a <code>#[shared]</code> resource that's only accessed by tasks
|
|
running at the <em>same</em> priority. In this case, you can opt out of the <code>lock</code> API by adding the
|
|
<code>#[lock_free]</code> field-level attribute to the resource declaration (see example below). Note that
|
|
this is merely a convenience to reduce needless resource locking code, because even if the
|
|
<code>lock</code> API is used, at runtime the framework will <strong>not</strong> produce a critical section due to how
|
|
the underlying resource-ceiling preemption works.</p>
|
|
<p>Also worth noting: using <code>#[lock_free]</code> on resources shared by
|
|
tasks running at different priorities will result in a <em>compile-time</em> error -- not using the <code>lock</code>
|
|
API would be a data race in that case.</p>
|
|
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
|
|
</span><span class="boring">fn main() {
|
|
</span>//! examples/lock-free.rs
|
|
|
|
#![deny(unsafe_code)]
|
|
#![deny(warnings)]
|
|
#![deny(missing_docs)]
|
|
#![no_main]
|
|
#![no_std]
|
|
|
|
use panic_semihosting as _;
|
|
|
|
#[rtic::app(device = lm3s6965, dispatchers = [GPIOA])]
|
|
mod app {
|
|
use cortex_m_semihosting::{debug, hprintln};
|
|
|
|
#[shared]
|
|
struct Shared {
|
|
#[lock_free] // <- lock-free shared resource
|
|
counter: u64,
|
|
}
|
|
|
|
#[local]
|
|
struct Local {}
|
|
|
|
#[init]
|
|
fn init(_: init::Context) -> (Shared, Local, init::Monotonics) {
|
|
foo::spawn().unwrap();
|
|
|
|
(Shared { counter: 0 }, Local {}, init::Monotonics())
|
|
}
|
|
|
|
#[task(shared = [counter])] // <- same priority
|
|
fn foo(c: foo::Context) {
|
|
bar::spawn().unwrap();
|
|
|
|
*c.shared.counter += 1; // <- no lock API required
|
|
let counter = *c.shared.counter;
|
|
hprintln!(" foo = {}", counter);
|
|
}
|
|
|
|
#[task(shared = [counter])] // <- same priority
|
|
fn bar(c: bar::Context) {
|
|
foo::spawn().unwrap();
|
|
|
|
*c.shared.counter += 1; // <- no lock API required
|
|
let counter = *c.shared.counter;
|
|
hprintln!(" bar = {}", counter);
|
|
|
|
debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
|
|
}
|
|
}
|
|
<span class="boring">}</span></code></pre></pre>
|
|
<pre><code class="language-console">$ cargo run --target thumbv7m-none-eabi --example lock-free
|
|
foo = 1
|
|
bar = 2
|
|
</code></pre>
|
|
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