rcu: Merge RCU-bh into RCU-preempt

The Linux kernel has long RCU-bh read-side critical sections that
intolerably increase scheduling latency under mainline's RCU-bh rules,
which include RCU-bh read-side critical sections being non-preemptible.
This patch therefore arranges for RCU-bh to be implemented in terms of
RCU-preempt for CONFIG_PREEMPT_RT_FULL=y.

This has the downside of defeating the purpose of RCU-bh, namely,
handling the case where the system is subjected to a network-based
denial-of-service attack that keeps at least one CPU doing full-time
softirq processing.  This issue will be fixed by a later commit.

The current commit will need some work to make it appropriate for
mainline use, for example, it needs to be extended to cover Tiny RCU.

[ paulmck: Added a useful changelog ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20111005185938.GA20403@linux.vnet.ibm.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 12 insertions, 0 deletions

diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index c5b98e565aee..24dcc713bd00 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -77,6 +77,7 @@ int debug_lockdep_rcu_enabled(void)
 }
 EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);
 
+#ifndef CONFIG_PREEMPT_RT_FULL
 /**
  * rcu_read_lock_bh_held() - might we be in RCU-bh read-side critical section?
  *
@@ -96,6 +97,7 @@ int rcu_read_lock_bh_held(void)
 	return in_softirq() || irqs_disabled();
 }
 EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
+#endif
 
 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 8ef8675a0fbb..3afb0fd77017 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -170,6 +170,7 @@ void rcu_sched_qs(int cpu)
 	rdp->passed_quiesce = 1;
 }
 
+#ifndef CONFIG_PREEMPT_RT_FULL
 void rcu_bh_qs(int cpu)
 {
 	struct rcu_data *rdp = &per_cpu(rcu_bh_data, cpu);
@@ -180,6 +181,7 @@ void rcu_bh_qs(int cpu)
 		trace_rcu_grace_period("rcu_bh", rdp->gpnum, "cpuqs");
 	rdp->passed_quiesce = 1;
 }
+#endif
 
 /*
  * Note a context switch.  This is a quiescent state for RCU-sched,
@@ -225,6 +227,7 @@ long rcu_batches_completed_sched(void)
 }
 EXPORT_SYMBOL_GPL(rcu_batches_completed_sched);
 
+#ifndef CONFIG_PREEMPT_RT_FULL
 /*
  * Return the number of RCU BH batches processed thus far for debug & stats.
  */
@@ -242,6 +245,7 @@ void rcu_bh_force_quiescent_state(void)
 	force_quiescent_state(&rcu_bh_state, 0);
 }
 EXPORT_SYMBOL_GPL(rcu_bh_force_quiescent_state);
+#endif
 
 /*
  * Record the number of times rcutorture tests have been initiated and
@@ -1667,6 +1671,7 @@ void call_rcu_sched(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 }
 EXPORT_SYMBOL_GPL(call_rcu_sched);
 
+#ifndef CONFIG_PREEMPT_RT_FULL
 /*
  * Queue an RCU for invocation after a quicker grace period.
  */
@@ -1675,6 +1680,7 @@ void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 	__call_rcu(head, func, &rcu_bh_state);
 }
 EXPORT_SYMBOL_GPL(call_rcu_bh);
+#endif
 
 /**
  * synchronize_sched - wait until an rcu-sched grace period has elapsed.
@@ -1707,6 +1713,7 @@ void synchronize_sched(void)
 }
 EXPORT_SYMBOL_GPL(synchronize_sched);
 
+#ifndef CONFIG_PREEMPT_RT_FULL
 /**
  * synchronize_rcu_bh - wait until an rcu_bh grace period has elapsed.
  *
@@ -1723,6 +1730,7 @@ void synchronize_rcu_bh(void)
 	wait_rcu_gp(call_rcu_bh);
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
+#endif
 
 /*
  * Check to see if there is any immediate RCU-related work to be done
@@ -1877,6 +1885,7 @@ static void _rcu_barrier(struct rcu_state *rsp,
 	mutex_unlock(&rcu_barrier_mutex);
 }
 
+#ifndef CONFIG_PREEMPT_RT_FULL
 /**
  * rcu_barrier_bh - Wait until all in-flight call_rcu_bh() callbacks complete.
  */
@@ -1885,6 +1894,7 @@ void rcu_barrier_bh(void)
 	_rcu_barrier(&rcu_bh_state, call_rcu_bh);
 }
 EXPORT_SYMBOL_GPL(rcu_barrier_bh);
+#endif
 
 /**
  * rcu_barrier_sched - Wait for in-flight call_rcu_sched() callbacks.