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Commit e90c53d3 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

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: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 5b61b0ba

kernel/rcutree.c

+0 −3
Original line number Diff line number Diff line
@@ -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");

}

kernel/rcutree.h

+0 −1
Original line number Diff line number Diff line
@@ -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);

kernel/rcutree_plugin.h

+0 −25
Original line number Diff line number Diff line
@@ -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) */