[PATCH] rcu: Is it safe to enter an RCU read-side critical section?

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

There is currently no way for kernel code to determine whether it
is safe to enter an RCU read-side critical section, in other words,
whether or not RCU is paying attention to the currently running CPU.
Given the large and increasing quantity of code shared by the idle loop
and non-idle code, the this shortcoming is becoming increasingly painful.

This commit therefore adds rcu_watching_this_cpu(), which returns true
if it is safe to enter an RCU read-side critical section on the currently
running CPU.  This function is quite fast, using only a __this_cpu_read().
However, the caller must disable preemption.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

 include/linux/rcupdate.h |    1 +
 kernel/rcutree.c         |   12 ++++++++++++
 2 files changed, 13 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 15d33d9..1c7112c 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -225,6 +225,7 @@ extern void rcu_idle_enter(void);
 extern void rcu_idle_exit(void);
 extern void rcu_irq_enter(void);
 extern void rcu_irq_exit(void);
+extern bool rcu_watching_this_cpu(void);
 
 #ifdef CONFIG_RCU_USER_QS
 extern void rcu_user_enter(void);
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index a06d172..7b8fcee 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -710,6 +710,18 @@ EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online);
 #endif /* #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU) */
 
 /**
+ * rcu_watching_this_cpu - are RCU read-side critical sections safe?
+ *
+ * Return true if RCU is watching the running CPU, which means that this
+ * CPU can safely enter RCU read-side critical sections.  The caller must
+ * have at least disabled preemption.
+ */
+bool rcu_watching_this_cpu(void)
+{
+	return !!__this_cpu_read(rcu_dynticks.dynticks_nesting);
+}
+
+/**
  * rcu_is_cpu_rrupt_from_idle - see if idle or immediately interrupted from idle
  *
  * If the current CPU is idle or running at a first-level (not nested)