```rust
use stm32_metapac as pac;

let lock_gpio =|gpio: pac::gpio::Gpio| {
    gpio.lckr.write_value(Lckr(0x1FFFF));
    gpio.lckr.write_value(Lckr(0x0FFFF));
    gpio.lckr.write_value(Lckr(0x1FFFF));
    gpio.lckr.read();
    if !gpio.lckr.read().lckk() {
        panic!();
    }
};

lock_gpio(pac::GPIOA);
lock_gpio(pac::GPIOB);
lock_gpio(pac::GPIOC);