core: assert only normal thread takes a mutex

Adds thread_is_in_normal_mode() which returns true if the current thread
is in normal mode. Adds an assert(thread_is_in_normal_mode()) in
__mutex_lock().

Tested-by: Jens Wiklander <jens.wiklander@linaro.org> (QEMU)
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

core/arch/arm/include/kernel/thread.h

@@ -497,6 +497,8 @@ vaddr_t thread_stack_start(void);
 /* Returns the stack size for the current thread */
 size_t thread_stack_size(void);
 
+bool thread_is_in_normal_mode(void);
+
 /*
  * Adds a mutex to the list of held mutexes for current thread
  * Requires foreign interrupts to be disabled.

core/arch/arm/kernel/mutex.c

@@ -40,6 +40,7 @@ static void __mutex_lock(struct mutex *m, const char *fname, int lineno)
 {
 	assert_have_no_spinlock();
 	assert(thread_get_id_may_fail() != -1);
+	assert(thread_is_in_normal_mode());
 
 	while (true) {
 		uint32_t old_itr_status;

core/arch/arm/kernel/thread.c

@@ -642,6 +642,28 @@ size_t thread_stack_size(void)
 	return STACK_THREAD_SIZE;
 }
 
+#ifdef ARM32
+bool thread_is_in_normal_mode(void)
+{
+	return (read_cpsr() & ARM32_CPSR_MODE_MASK) == ARM32_CPSR_MODE_SVC;
+}
+#endif
+
+#ifdef ARM64
+bool thread_is_in_normal_mode(void)
+{
+	uint32_t exceptions = thread_mask_exceptions(THREAD_EXCP_FOREIGN_INTR);
+	struct thread_core_local *l = thread_get_core_local();
+	bool ret;
+
+	/* If any bit in l->flags is set we're handling some exception. */
+	ret = !l->flags;
+	thread_unmask_exceptions(exceptions);
+
+	return ret;
+}
+#endif
+
 void thread_state_free(void)
 {
 	struct thread_core_local *l = thread_get_core_local();