rtic/examples/preemption.rs

//! Two tasks running at *different* priorities with access to the same resource
#![deny(unsafe_code)]
#![deny(warnings)]
#![feature(proc_macro)]
#![no_std]

extern crate cortex_m_rtfm as rtfm;
extern crate stm32f103xx;

use rtfm::{app, Resource, Threshold};

app! {
    device: stm32f103xx,

    resources: {
        static COUNTER: u64 = 0;
    },

    tasks: {
        // The `SYS_TICK` task has higher priority than `TIM2`
        SYS_TICK: {
            path: sys_tick,
            priority: 2,
            resources: [COUNTER],
        },

        TIM2: {
            path: tim2,
            priority: 1,
            resources: [COUNTER],
        },
    },
}

fn init(_p: init::Peripherals, _r: init::Resources) {
    // ..
}

fn idle() -> ! {
    loop {
        rtfm::wfi();
    }
}

fn sys_tick(_t: &mut Threshold, r: SYS_TICK::Resources) {
    // ..

    // This task can't be preempted by `tim2` so it has direct access to the
    // resource data
    **r.COUNTER += 1;

    // ..
}

fn tim2(t: &mut Threshold, mut r: TIM2::Resources) {
    // ..

    // As this task runs at lower priority it needs a critical section to
    // prevent `sys_tick` from preempting it while it modifies this resource
    // data. The critical section is required to prevent data races which can
    // lead to undefined behavior.
    r.COUNTER.claim_mut(t, |counter, _t| {
        // `claim_mut` creates a critical section
        **counter += 1;
    });

    // ..
}