Sharing SPI between tasks

The code

Embedded C for Cortex-M. It compiles. There are 3 real bugs in it: the kind that pass a bench test and fail in the field.

SemaphoreHandle_t spi_lock;

void app_init(void)
{
    spi_lock = xSemaphoreCreateBinary();
    xSemaphoreGive(spi_lock);
}

void display_task(void *p)    /* priority 3 */
{
    for (;;) {
        xSemaphoreTake(spi_lock, portMAX_DELAY);
        lcd_flush();
        xSemaphoreGive(spi_lock);
    }
}

void logger_task(void *p)     /* priority 1 */
{
    for (;;) {
        xSemaphoreTake(spi_lock, portMAX_DELAY);
        sd_write_block();
        if (sd_error()) continue;
        xSemaphoreGive(spi_lock);
        vTaskDelay(pdMS_TO_TICKS(100));
    }
}

What an interviewer expects

• Point to each line you'd stop in a code review.
• Say what actually breaks: the failure mode, not just the rule it violates.
• Fix it, then defend the fix when asked the follow-up.

The bugs, reference fixes and the follow-up question are in the practice app, after you've tried it.

Companies that test this

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