rcu: Detect illegal rcu dereference in extended quiescent state

[linux-beck.git] / include / linux / rcupdate.h

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 8d315b013e37371238a3daac82aa9f7f3e2d4725..bf91fcfe181cee9b32dc5d49548ccc44f8ce3001 100644 (file)
--- a/include/linux/rcupdate.h
+++ b/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();