rcu: Remove rcu_needs_cpu_flush() to avoid false quiescent states

The purpose of rcu_needs_cpu_flush() was to iterate on pushing the
current grace period in order to help the current CPU enter dyntick-idle
mode.  However, this can result in failures if the CPU starts entering
dyntick-idle mode, but then backs out.  In this case, the call to
rcu_pending() from rcu_needs_cpu_flush() might end up announcing a
non-existing quiescent state.

This commit therefore removes rcu_needs_cpu_flush() in favor of letting
the dyntick-idle machinery at the end of the softirq handler push the
loop along via its call to rcu_pending().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 8455043..e75df0c 100644 (file)
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1528,9 +1528,6 @@ static void rcu_process_callbacks(struct softirq_action *unused)
                                &__get_cpu_var(rcu_sched_data));
        __rcu_process_callbacks(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
        rcu_preempt_process_callbacks();
-
-       /* If we are last CPU on way to dyntick-idle mode, accelerate it. */
-       rcu_needs_cpu_flush();
        trace_rcu_utilization("End RCU core");
 }
 

diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index f509f72..849ce9e 100644 (file)
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -458,7 +458,6 @@ static int rcu_preempt_needs_cpu(int cpu);
 static void __cpuinit rcu_preempt_init_percpu_data(int cpu);
 static void rcu_preempt_send_cbs_to_online(void);
 static void __init __rcu_init_preempt(void);
-static void rcu_needs_cpu_flush(void);
 static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags);
 static void rcu_preempt_boost_start_gp(struct rcu_node *rnp);
 static void invoke_rcu_callbacks_kthread(void);

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index b4cbe5b..4b9b9f8 100644 (file)
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1948,15 +1948,6 @@ int rcu_needs_cpu(int cpu)
        return rcu_needs_cpu_quick_check(cpu);
 }
 
-/*
- * Check to see if we need to continue a callback-flush operations to
- * allow the last CPU to enter dyntick-idle mode.  But fast dyntick-idle
- * entry is not configured, so we never do need to.
- */
-static void rcu_needs_cpu_flush(void)
-{
-}
-
 #else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
 
 #define RCU_NEEDS_CPU_FLUSHES 5
@@ -2032,20 +2023,4 @@ int rcu_needs_cpu(int cpu)
        return c;
 }
 
-/*
- * Check to see if we need to continue a callback-flush operations to
- * allow the last CPU to enter dyntick-idle mode.
- */
-static void rcu_needs_cpu_flush(void)
-{
-       int cpu = smp_processor_id();
-       unsigned long flags;
-
-       if (per_cpu(rcu_dyntick_drain, cpu) <= 0)
-               return;
-       local_irq_save(flags);
-       (void)rcu_needs_cpu(cpu);
-       local_irq_restore(flags);
-}
-
 #endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */