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

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_is_watching(), 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>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index e31005ee339e..bee665964878 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -132,4 +132,13 @@ static inline void rcu_scheduler_starting(void)
 }
 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
+#ifdef CONFIG_RCU_TRACE
+
+static inline bool __rcu_is_watching(void)
+{
+	return !rcu_is_cpu_idle();
+}
+
+#endif /* #ifdef CONFIG_RCU_TRACE */
+
 #endif /* __LINUX_RCUTINY_H */