c
// Introduce a reference count in the spi_controller structure
struct spi_controller {
    // ...
    int refcount;
    // ...
};

// Increment the reference count before locking the mutex
5  spi->controller->refcount++;
6  mutex_lock(&spi->controller->bus_lock_mutex);

// Decrement the reference count after unlocking the mutex
7  mutex_unlock(&spi->controller->bus_lock_mutex);
8  spi->controller->refcount--;

// When freeing the spi_controller, check the reference count
void free_spi_controller(struct spi_controller *controller) {
    if (controller->refcount == 0) {
        // Safe to free the controller
        kfree(controller);
    }
}
