RP2040: USB CDC output drops characters with `-scheduler=cores` on TinyGo 0.41.1

[rdon-key]

Summary

On Raspberry Pi Pico / RP2040, USB CDC output appears to drop characters when using -scheduler=cores.

The same program prints correctly with TinyGo 0.40.1, but the output becomes corrupted with TinyGo 0.41.1.

Environment

• Board: Raspberry Pi Pico / RP2040
• Host OS: Windows 11
• CPU: AMD Ryzen 5 5600G
• Shell: Git Bash / MINGW64
• Install method: Scoop
• USB connection: onboard USB CDC
• Serial monitor: tinygo flash -monitor
• TinyGo 0.40.1:
  • tinygo version 0.40.1 windows/amd64 (using go version go1.24.13 and LLVM version 20.1.1)
• TinyGo 0.41.1:
  • tinygo version 0.41.1 windows/amd64 (using go version go1.24.13 and LLVM version 20.1.1)

Reproducer

package main

import "time"

func main() {
	time.Sleep(2 * time.Second)

	println("start usb only")

	for i := 0; i < 100; i++ {
		if i == 0 || i%10 == 0 {
			println("rounds ok:", i, "/ 100")
		}
		time.Sleep(10 * time.Millisecond)
	}

	println("test finished")
}

Commands

TinyGo 0.40.1:

scoop reset tinygo@0.40.1
C:/Users/user/scoop/shims/tinygo.exe flash -target=pico -scheduler=cores -monitor main_usb_only.go

TinyGo 0.41.1:

scoop reset tinygo@0.41.1
C:/Users/user/scoop/shims/tinygo.exe flash -target=pico -scheduler=cores -monitor main_usb_only.go

I also checked -scheduler=tasks with TinyGo 0.41.1:

C:/Users/user/scoop/shims/tinygo.exe flash -target=pico -scheduler=tasks -monitor main_usb_only.go

Observed behavior

With TinyGo 0.40.1 and -scheduler=cores, the output appears correct:

Connected to COM3. Press Ctrl-C to exit.
start usb only
rounds ok: 0 / 100
rounds ok: 10 / 100
rounds ok: 20 / 100
rounds ok: 30 / 100
rounds ok: 40 / 100
rounds ok: 50 / 100
rounds ok: 60 / 100
rounds ok: 70 / 100
rounds ok: 80 / 100
rounds ok: 90 / 100
test finished

With TinyGo 0.41.1 and -scheduler=cores, the output drops characters:

Connected to COM3. Press Ctrl-C to exit.
ds ok:  100y
rounds ok: 10 / 100
rounds ok: 20 / 100
rounds ok: 30 / 100
rk: 40 / 100
rou 100
rnds ok: 60
rounds ok: 70 / 100
rounds ok: 80 / 100
r100
test finished

With TinyGo 0.41.1 and -scheduler=tasks, the same program prints correctly:

Connected to COM3. Press Ctrl-C to exit.
start usb only
rounds ok: 0 / 100
rounds ok: 10 / 100
rounds ok: 20 / 100
rounds ok: 30 / 100
rounds ok: 40 / 100
rounds ok: 50 / 100
rounds ok: 60 / 100
rounds ok: 70 / 100
rounds ok: 80 / 100
rounds ok: 90 / 100
test finished

Expected behavior

USB CDC output should not drop characters when using -scheduler=cores.

The output should be consistent with the behavior of TinyGo 0.40.1 and with -scheduler=tasks.