clock_systimer: 64-bit timer accesses are not atomic. Noted by Freddie Chopin.

sched/clock/Make.defs

@@ -1,7 +1,7 @@
 ############################################################################
 # sched/clock/Make.defs
 #
-#   Copyright (C) 2014 Gregory Nutt. All rights reserved.
+#   Copyright (C) 2014, 2016 Gregory Nutt. All rights reserved.
 #   Author: Gregory Nutt <gnutt@nuttx.org>
 #
 # Redistribution and use in source and binary forms, with or without

sched/clock/clock_systimer.c

@@ -80,7 +80,8 @@
 systime_t clock_systimer(void)
 {
 #ifdef CONFIG_SCHED_TICKLESS
-#ifdef CONFIG_SYSTEM_TIME64
+# ifdef CONFIG_SYSTEM_TIME64
+
   struct timespec ts;
 
   /* Get the time from the platform specific hardware */
@@ -90,7 +91,9 @@ systime_t clock_systimer(void)
   /* Convert to a 64-bit value in microseconds, then in clock tick units */
 
   return USEC2TICK(1000000 * (uint64_t)ts.tv_sec + (uint64_t)ts.tv_nsec / 1000);
-#else
+
+# else /* CONFIG_SYSTEM_TIME64 */
+
   struct timespec ts;
   uint64_t tmp;
 
@@ -98,16 +101,31 @@ systime_t clock_systimer(void)
 
   (void)up_timer_gettime(&ts);
 
-  /* Convert to a 64- then 32-bit value */
+  /* Convert to a 64- then a 32-bit value */
 
   tmp = MSEC2TICK(1000 * (uint64_t)ts.tv_sec + (uint64_t)ts.tv_nsec / 1000000);
   return (systime_t)(tmp & 0x00000000ffffffff);
-#endif
-#else
+
+# endif /* CONFIG_SYSTEM_TIME64 */
+#else /* CONFIG_SCHED_TICKLESS */
+# ifdef CONFIG_SYSTEM_TIME64
+
+  irqstate_t flags;
+  systime_t sample;
+
+  /* 64-bit accesses are not atomic on most architectures. */
+
+  flags  = irqsave();
+  sample = g_system_timer;
+  irqrestore(flags);
+  return sample;
+
+# else /* CONFIG_SYSTEM_TIME64 */
 
   /* Return the current system time */
 
   return g_system_timer;
 
-#endif
+# endif /* CONFIG_SYSTEM_TIME64 */
+#endif /* CONFIG_SCHED_TICKLESS */
 }