freertos global variable inside isr

Question

I'm new to RTOS and having some troubles understanding a strange behavior:

I have a STM32 microcontroller running FreeRTOS and an RTC interrupt running, too. The RTC interrupt just updates a volatile uint32_t variable named SystemTime:

void HAL_RTC_AlarmAEventCallback(RTC_HandleTypeDef *hrtc) {
  UNUSED(hrtc);
  SystemTime++;
}

Additionally, I have another task that runs every 100 mS. It just prints the SystemTime value if it has changed.

static void ToggleLEDThread(void const *argument) {
  (void) argument;
  static uint32_t ost;
  uint32_t tst;

  for (;;) {
#if 0
    tst = SystemTime;
    if (ost != tst) {
      xprintf("<%d>\n", tst);
      ost = tst;
    }
#else
    if (ost != SystemTime) {
      xprintf("<%d>\n", SystemTime);
      ost = SystemTime;
    }
#endif
    vTaskDelay(100);
  }
}

It works as expected if #if 1 (using a temporary variable), but when #if 0, the code runs ok for some time before it stops printing and after a few seconds starts to print again.

There is another task that prints some other values once a second.

Output when it's working:

V:1139  O:1091
<35>
V:1139  O:1123
<36>
V:1140  O:1154
<37>
V:1140  O:1186
<38>
V:1139  O:1218
<39>
V:1139  O:1249
<40>
V:1139  O:1281
<41>
V:1139  O:1313
<42>
V:1139  O:1344
<43>
V:1139  O:1376
<44>
V:1139  O:1408
<45>
V:1139  O:1439
<46>
V:1140  O:1471
<47>
V:1139  O:1503
<48>
V:1139  O:1535
<49>
V:1139  O:1566
<50>
V:1140  O:1598
<51>
V:1139  O:1630
<52>
V:1139  O:1661

Output when the problem happens:

V:1139  O:1091
<35>
V:1139  O:1123
<36>
V:1140  O:1154
<37>
V:1140  O:1186
<38>
V:1139  O:1218
<39>
V:1139  O:1249
<40>
V:1139  O:1281
<41>
V:1139  O:1313
<42>
V:1139  O:1344
V:1139  O:1376
V:1139  O:1408
V:1139  O:1439
V:1140  O:1471
V:1139  O:1503
V:1139  O:1535
V:1139  O:1566
<50>
V:1140  O:1598
<51>
V:1139  O:1630
<52>
V:1139  O:1661

Any ideas?

Answer 1

if (ost != SystemTime) {
    xprintf("<%d>\n", SystemTime);
    ost = SystemTime;
}

Here. While you are printing your SystemTimeValue, interrupt occur and your value is incremented. After that you equate a variable ost to modified variable SystemTime. Thats why next round is missing and code does not printing new time value.

Just a guess