From 1c367a2a1ce8b02a2d7af23e10823a6a86460a3d Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <fweisbec@gmail.com>
Date: Fri, 7 Oct 2011 16:25:18 -0700
Subject: [PATCH] --- yaml --- r: 277202 b: refs/heads/master c:
 e6b80a3b0994ea6c3d876d72464f2debbfcfeb05 h: refs/heads/master v: v3

---
 [refs]                         |  2 +-
 trunk/include/linux/rcupdate.h | 26 ++++++++++++++++++++++++++
 trunk/kernel/rcupdate.c        |  2 ++
 trunk/kernel/rcutiny.c         |  1 +
 trunk/kernel/rcutree.c         |  1 +
 5 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/[refs] b/[refs]
index 458c128cba1b..1ea1016a5fac 100644
--- a/[refs]
+++ b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: a0f8eefb127f5be07628954f310a7fc8c82b2fc3
+refs/heads/master: e6b80a3b0994ea6c3d876d72464f2debbfcfeb05
diff --git a/trunk/include/linux/rcupdate.h b/trunk/include/linux/rcupdate.h
index 8d315b013e37..bf91fcfe181c 100644
--- a/trunk/include/linux/rcupdate.h
+++ b/trunk/include/linux/rcupdate.h
@@ -228,6 +228,15 @@ static inline void destroy_rcu_head_on_stack(struct rcu_head *head)
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 
+#ifdef CONFIG_PROVE_RCU
+extern int rcu_is_cpu_idle(void);
+#else /* !CONFIG_PROVE_RCU */
+static inline int rcu_is_cpu_idle(void)
+{
+	return 0;
+}
+#endif /* else !CONFIG_PROVE_RCU */
+
 extern struct lockdep_map rcu_lock_map;
 # define rcu_read_acquire() \
 		lock_acquire(&rcu_lock_map, 0, 0, 2, 1, NULL, _THIS_IP_)
@@ -262,6 +271,8 @@ static inline int rcu_read_lock_held(void)
 {
 	if (!debug_lockdep_rcu_enabled())
 		return 1;
+	if (rcu_is_cpu_idle())
+		return 0;
 	return lock_is_held(&rcu_lock_map);
 }
 
@@ -285,6 +296,19 @@ extern int rcu_read_lock_bh_held(void);
  *
  * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
  * and while lockdep is disabled.
+ *
+ * Note that if the CPU is in the idle loop from an RCU point of
+ * view (ie: that we are in the section between rcu_idle_enter() and
+ * rcu_idle_exit()) then rcu_read_lock_held() returns false even if the CPU
+ * did an rcu_read_lock().  The reason for this is that RCU ignores CPUs
+ * that are in such a section, considering these as in extended quiescent
+ * state, so such a CPU is effectively never in an RCU read-side critical
+ * section regardless of what RCU primitives it invokes.  This state of
+ * affairs is required --- we need to keep an RCU-free window in idle
+ * where the CPU may possibly enter into low power mode. This way we can
+ * notice an extended quiescent state to other CPUs that started a grace
+ * period. Otherwise we would delay any grace period as long as we run in
+ * the idle task.
  */
 #ifdef CONFIG_PREEMPT_COUNT
 static inline int rcu_read_lock_sched_held(void)
@@ -293,6 +317,8 @@ static inline int rcu_read_lock_sched_held(void)
 
 	if (!debug_lockdep_rcu_enabled())
 		return 1;
+	if (rcu_is_cpu_idle())
+		return 0;
 	if (debug_locks)
 		lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
 	return lockdep_opinion || preempt_count() != 0 || irqs_disabled();
diff --git a/trunk/kernel/rcupdate.c b/trunk/kernel/rcupdate.c
index 92e771d7b44b..2bc4e135ff23 100644
--- a/trunk/kernel/rcupdate.c
+++ b/trunk/kernel/rcupdate.c
@@ -93,6 +93,8 @@ int rcu_read_lock_bh_held(void)
 {
 	if (!debug_lockdep_rcu_enabled())
 		return 1;
+	if (rcu_is_cpu_idle())
+		return 0;
 	return in_softirq() || irqs_disabled();
 }
 EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
diff --git a/trunk/kernel/rcutiny.c b/trunk/kernel/rcutiny.c
index 3ab77bdc90c4..b4e0b4981768 100644
--- a/trunk/kernel/rcutiny.c
+++ b/trunk/kernel/rcutiny.c
@@ -157,6 +157,7 @@ int rcu_is_cpu_idle(void)
 {
 	return !rcu_dynticks_nesting;
 }
+EXPORT_SYMBOL(rcu_is_cpu_idle);
 
 #endif /* #ifdef CONFIG_PROVE_RCU */
 
diff --git a/trunk/kernel/rcutree.c b/trunk/kernel/rcutree.c
index 8afb2e89745b..489b62a67d35 100644
--- a/trunk/kernel/rcutree.c
+++ b/trunk/kernel/rcutree.c
@@ -567,6 +567,7 @@ int rcu_is_cpu_idle(void)
 	preempt_enable();
 	return ret;
 }
+EXPORT_SYMBOL(rcu_is_cpu_idle);
 
 #endif /* #ifdef CONFIG_PROVE_RCU */