rcu: Enable DEBUG_OBJECTS_RCU_HEAD from !PREEMPT

The prohibition of DEBUG_OBJECTS_RCU_HEAD from !PREEMPT was due to the
fixup actions.  So just produce a warning from !PREEMPT.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/rcupdate.c

@@ -142,7 +142,14 @@ static int rcuhead_fixup_init(void *addr, enum debug_obj_state state)
 		 * Ensure that queued callbacks are all executed.
 		 * If we detect that we are nested in a RCU read-side critical
 		 * section, we should simply fail, otherwise we would deadlock.
+		 * In !PREEMPT configurations, there is no way to tell if we are
+		 * in a RCU read-side critical section or not, so we never
+		 * attempt any fixup and just print a warning.
 		 */
+#ifndef CONFIG_PREEMPT
+		WARN_ON(1);
+		return 0;
+#endif
 		if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
 		    irqs_disabled()) {
 			WARN_ON(1);
@@ -184,7 +191,14 @@ static int rcuhead_fixup_activate(void *addr, enum debug_obj_state state)
 		 * Ensure that queued callbacks are all executed.
 		 * If we detect that we are nested in a RCU read-side critical
 		 * section, we should simply fail, otherwise we would deadlock.
+		 * In !PREEMPT configurations, there is no way to tell if we are
+		 * in a RCU read-side critical section or not, so we never
+		 * attempt any fixup and just print a warning.
 		 */
+#ifndef CONFIG_PREEMPT
+		WARN_ON(1);
+		return 0;
+#endif
 		if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
 		    irqs_disabled()) {
 			WARN_ON(1);
@@ -214,12 +228,14 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state)
 		 * Ensure that queued callbacks are all executed.
 		 * If we detect that we are nested in a RCU read-side critical
 		 * section, we should simply fail, otherwise we would deadlock.
-		 * Note that the machinery to reliably determine whether
-		 * or not we are in an RCU read-side critical section
-		 * exists only in the preemptible RCU implementations
-		 * (TINY_PREEMPT_RCU and TREE_PREEMPT_RCU), which is why
-		 * DEBUG_OBJECTS_RCU_HEAD is disallowed if !PREEMPT.
+		 * In !PREEMPT configurations, there is no way to tell if we are
+		 * in a RCU read-side critical section or not, so we never
+		 * attempt any fixup and just print a warning.
 		 */
+#ifndef CONFIG_PREEMPT
+		WARN_ON(1);
+		return 0;
+#endif
 		if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
 		    irqs_disabled()) {
 			WARN_ON(1);

lib/Kconfig.debug

@@ -337,7 +337,7 @@ config DEBUG_OBJECTS_WORK
 
 config DEBUG_OBJECTS_RCU_HEAD
 	bool "Debug RCU callbacks objects"
-	depends on DEBUG_OBJECTS && PREEMPT
+	depends on DEBUG_OBJECTS
 	help
 	  Enable this to turn on debugging of RCU list heads (call_rcu() usage).