sys/xtimer: Avoid race incrementing multiple periods in _timer_callback

On a fast CPU with a slow timer (e.g. XTIMER_SHIFT > 0) it is possible
that now == _xtimer_now() when spinning for the overflow. In the extreme
case When this happens _next_period() will be called more than once
until the timer overflows for real.

Fault observed in real life when running on a 32.768 kHz timer on a
~96 MHz clocked mulle (Kinetis K60, Cortex-M4). _next_period() was
called 9 times during the same ISR call before the 32 kHz timer
Joakim Nohlgård 8 years ago
parent d18063cbe1
commit b365ab45cb

@ -499,7 +499,7 @@ overflow:
/* check if the end of this period is very soon */
if (_mask(now + XTIMER_ISR_BACKOFF) < now) {
/* spin until next period, then advance */
while (_xtimer_now() > now);
while (_xtimer_now() >= now);
reference = 0;
goto overflow;