Output corrupted when using multiple rtt_target channels

[funmaker]

Hi. I am using defmt on top of rtt_target on esp32s3, launched by probe-rs. I am trying to set up multiple rtt channels, one for defmt and another for other information. However as soon as I declare the existence of another up channel in rtt_target::rtt_init! macro, defmt output seems corrupted.

My set up looks like this:

#[esp_hal_embassy::main]
async fn main(spawner: Spawner) {
    let mut channels = rtt_target::rtt_init! {
        up: {
            0: { size: 1024, mode: ChannelMode::NoBlockSkip, name: "Terminal" }
            1: { size: 1024, name: "Perf" }
        }
        down: {
            0: { size: 16, name: "Terminal" }
        }
    };
    
    rtt_target::set_defmt_channel(channels.up.0);

    defmt::info!("Hello from defmt.");
    channels.up.1.write(b"Hello from Channel 2\n");
}

Output looks like this:

[Brminal] ~~    ~
16:03:54.429: ~[Perf    ] Hello from Channel 2

But if I comment out the 2nd channel and 2nd print message, it all works as expected:

[INFO ] Hello from defmt.

I run probe-rs with:

$ probe-rs run --chip=esp32s3 --preverify --always-print-stacktrace --no-location --catch-hardfault /path/to/target/debug/program

Edit: it seems like there is some data race going on. Sometimes, the output works, sometimes not. Sometimes, adding seemingly noop code makes it work, and sometimes not. Eg adding if core::hint::black_box(1 == 0) { defmt::println!("{}", String::new()); } in some random function suddenly made it works, but then adding another line broke it again.

[jonathanpallant]

To be honest, this sounds like an issue with the rtt_target crate rather than the defmt crate. The defmt crate relies on a fairly simple external "C" API, which we implement with our own defmt_rtt and defmt_semihosting crates, but other crates are free to implement as well.