Nested mutexes under load

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_mtx, log_mtx;
QueueHandle_t rec_q;

void log_write(const char *msg)
{
    xSemaphoreTake(log_mtx, portMAX_DELAY);
    log_append(msg);
    if (log_full()) {
        xSemaphoreTake(spi_mtx, portMAX_DELAY);
        flush_log_to_flash();
        xSemaphoreGive(spi_mtx);
    }
    xSemaphoreGive(log_mtx);
}

void storage_task(void *p)      /* priority 1 */
{
    for (;;) {
        rec_t r;
        xSemaphoreTake(spi_mtx, portMAX_DELAY);
        xQueueReceive(rec_q, &r, portMAX_DELAY);
        flash_write_record(&r);
        log_write("record stored");
        xSemaphoreGive(spi_mtx);
    }
}

void control_task(void *p)      /* priority 5, 1 ms period */
{
    for (;;) {
        run_control();
        if (fault()) log_write("fault");
        vTaskDelay(1);
    }
}

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.

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