kernel: add k_is_preempt_thread()

Useful for finding out if the current thread is protected against
preemption when using non-preemption to protect data structures.

Change-Id: Ib545a3609af3646ba49eeeb5a2c50dc51af010d4
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>

include/kernel.h

@@ -420,6 +420,20 @@ extern void k_sched_time_slice_set(int32_t slice, int prio);
  */
 extern int k_is_in_isr(void);
 
+/**
+ * @brief Determine if code is running in a preemptible thread.
+ *
+ * Returns a 'true' value if these conditions are all met:
+ *
+ * - the code is not running in an ISR
+ * - the thread's priority is in the preemptible range
+ * - the thread has not locked the scheduler
+ *
+ * @return 0 if invoked by either an ISR or a cooperative thread.
+ * @return Non-zero if invoked by a preemptible thread.
+ */
+extern int k_is_preempt_thread(void);
+
 /*
  * @brief Lock the scheduler
  *

kernel/unified/sched.c

@@ -363,3 +363,8 @@ void k_sched_time_slice_set(int32_t duration_in_ms, int prio)
 	_time_slice_prio_ceiling = prio;
 }
 #endif /* CONFIG_TIMESLICING */
+
+int k_is_preempt_thread(void)
+{
+	return !_is_in_isr() && _is_preempt(_current);
+}