From b365ab45cbdfd953efb3689b48a1ba06875da808 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Joakim=20Nohlg=C3=A5rd?= <joakim.gebart@eistec.se>
Date: Wed, 30 Sep 2015 10:21:33 +0200
Subject: [PATCH] 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
overflowed.
---
sys/xtimer/xtimer_core.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/sys/xtimer/xtimer_core.c b/sys/xtimer/xtimer_core.c
index b882dd8f74..97b336f0c4 100644
--- a/sys/xtimer/xtimer_core.c
+++ b/sys/xtimer/xtimer_core.c
@@ -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);
_next_period();
reference = 0;
goto overflow;
--
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