[patch] RCU: introduce rcu_soon_pending() interface

[Heiko Carstens <heiko.carstens@de.ibm.com>]

From: Heiko Carstens <heiko.carstens@de.ibm.com>

Introduce rcu_soon_pending() interface. This can be used to tell if there
will be a new rcu batch on a cpu soon by looking at the curlist pointer.
This can be used to avoid to enter a tickless idle state where the cpu
would miss that a new batch is ready when rcu_start_batch would be called
on a different cpu.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
---

See also here: http://www.ussg.iu.edu/hypermail/linux/kernel/0604.3/0057.html
Better solutions welcome :)

 include/linux/rcupdate.h |    1 +
 kernel/rcupdate.c        |    8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 5673008..80bcbce 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -132,6 +132,7 @@ static inline void rcu_bh_qsctr_inc(int 
 }
 
 extern int rcu_pending(int cpu);
+extern int rcu_soon_pending(int cpu);
 
 /**
  * rcu_read_lock - mark the beginning of an RCU read-side critical section.
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index 13458bb..e8cf09f 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -485,6 +485,14 @@ int rcu_pending(int cpu)
 		__rcu_pending(&rcu_bh_ctrlblk, &per_cpu(rcu_bh_data, cpu));
 }
 
+int rcu_soon_pending(int cpu)
+{
+	struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
+	struct rcu_data *rdp_bh = &per_cpu(rcu_bh_data, cpu);
+
+	return (!!rdp->curlist || !!rdp_bh->curlist);
+}
+
 void rcu_check_callbacks(int cpu, int user)
 {
 	if (user ||