rcu: add support for dynamic ticks and preempt rcu

The PREEMPT-RCU can get stuck if a CPU goes idle and NO_HZ is set. The
idle CPU will not progress the RCU through its grace period and a
synchronize_rcu my get stuck. Without this patch I have a box that will
not boot when PREEMPT_RCU and NO_HZ are set. That same box boots fine
with this patch.

This patch comes from the -rt kernel where it has been tested for
several months.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 22 insertions, 0 deletions

diff --git a/include/linux/rcupreempt.h b/include/linux/rcupreempt.h
index 60c2a033b19e..01152ed532c8 100644
--- a/include/linux/rcupreempt.h
+++ b/include/linux/rcupreempt.h
@@ -82,5 +82,27 @@ extern struct rcupreempt_trace *rcupreempt_trace_cpu(int cpu);
 
 struct softirq_action;
 
+#ifdef CONFIG_NO_HZ
+DECLARE_PER_CPU(long, dynticks_progress_counter);
+
+static inline void rcu_enter_nohz(void)
+{
+        __get_cpu_var(dynticks_progress_counter)++;
+        WARN_ON(__get_cpu_var(dynticks_progress_counter) & 0x1);
+        mb();
+}
+
+static inline void rcu_exit_nohz(void)
+{
+        mb();
+        __get_cpu_var(dynticks_progress_counter)++;
+        WARN_ON(!(__get_cpu_var(dynticks_progress_counter) & 0x1));
+}
+
+#else /* CONFIG_NO_HZ */
+#define rcu_enter_nohz()        do { } while (0)
+#define rcu_exit_nohz()         do { } while (0)
+#endif /* CONFIG_NO_HZ */
+
 #endif /* __KERNEL__ */
 #endif /* __LINUX_RCUPREEMPT_H */