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rcu: Remove rcu_needs_cpu_flush() to avoid false quiescent states

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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>
This commit is contained in:
Paul E. McKenney 2011-08-20 18:29:32 -07:00
parent 5b61b0baa9
commit e90c53d3e2
3 changed files with 0 additions and 29 deletions
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3
kernel/rcutree.c
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@ -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");
}

1
kernel/rcutree.h
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@ -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);

25
kernel/rcutree_plugin.h
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@ -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) */