[PATCH] debug rcu head support !PREEMPT config

[Mathieu Desnoyers]

Remove DEBUG_RCU_HEAD dependency on PREEMPT config. Handle the unability to
detect if within a RCU read-side critical section by never performing any
attempt to recover from a failure situation in the fixup handlers. Just print
the warnings.

This patch is only compile-tested.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
---
 kernel/rcupdate.c |   17 +++++++++++++++++
 lib/Kconfig.debug |    2 +-
 2 files changed, 18 insertions(+), 1 deletion(-)

Index: linux-2.6-lttng/lib/Kconfig.debug
===================================================================
--- linux-2.6-lttng.orig/lib/Kconfig.debug
+++ linux-2.6-lttng/lib/Kconfig.debug
@@ -313,7 +313,7 @@ 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).
 
Index: linux-2.6-lttng/kernel/rcupdate.c
===================================================================
--- linux-2.6-lttng.orig/kernel/rcupdate.c
+++ linux-2.6-lttng/kernel/rcupdate.c
@@ -142,7 +142,14 @@ static int rcuhead_fixup_init(void *addr
 		 * 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 *
 		 * 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);
@@ -214,6 +228,9 @@ static int rcuhead_fixup_free(void *addr
 		 * 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);
-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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