Re: [RFC PATCH] rcu: don't ignore preempt_disable() in the idle loop

[Lai Jiangshan]

Steven Rostedt wrote:
> On Tue, 2010-03-09 at 19:13 +0800, Lai Jiangshan wrote:
>> Current, synchronize_sched() ignores preempt-disable()
>> sequences in the idle loop. It makes synchronize_sched()
>> is not so pure, and it hurts tracing.
>>
>> Paul have a proposal before:
>> http://lkml.org/lkml/2009/4/5/140
>> http://lkml.org/lkml/2009/4/6/496
>> But old fix needs to hack into all architectures' idle loops.
>>
>> This is another try, it uses the fact that idle loops
>> are executing with preept_count()=1.
>> But I didn't look deep into all idle loops.
> 
> Lai,
> 
> Does this (with your patch) fix the bug you were seeing with the ring
> buffer code?
> 

No, this can not fix the bug we found with the ring buffer code.
I think the bug is not come from this issue or from RCU.

Lai

> 
>> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
>> ---
>> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
>> index 3ec8160..0761723 100644
>> --- a/kernel/rcutree.c
>> +++ b/kernel/rcutree.c
>> @@ -80,6 +80,10 @@ DEFINE_PER_CPU(struct rcu_data, rcu_sched_data);
>>  struct rcu_state rcu_bh_state = RCU_STATE_INITIALIZER(rcu_bh_state);
>>  DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);
>>  
>> +#ifndef IDLE_CORE_LOOP_PREEMPT_COUNT
>> +#define IDLE_CORE_LOOP_PREEMPT_COUNT (1)
>> +#endif
>> +
>>  /*
>>   * Return true if an RCU grace period is in progress.  The ACCESS_ONCE()s
>>   * permit this function to be invoked without holding the root rcu_node
>> @@ -1114,6 +1118,26 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
>>  		raise_softirq(RCU_SOFTIRQ);
>>  }
>>  
>> +static inline int rcu_idle_qs(int cpu)
>> +{
>> +	if (!idle_cpu(cpu))
>> +		return 0;
>> +
>> +	if (!rcu_scheduler_active)
>> +		return 0;
>> +
>> +	if (in_softirq())
>> +		return 0;
>> +
>> +	if (hardirq_count() > (1 << HARDIRQ_SHIFT))
>> +		return 0;
>> +
>> +	if ((preempt_count() & PREEMPT_MASK) > IDLE_CORE_LOOP_PREEMPT_COUNT)
>> +		return 0;
>> +
>> +	return 1;
>> +}
>> +
>>  /*
>>   * Check to see if this CPU is in a non-context-switch quiescent state
>>   * (user mode or idle loop for rcu, non-softirq execution for rcu_bh).
>> @@ -1127,9 +1151,7 @@ void rcu_check_callbacks(int cpu, int user)
>>  {
>>  	if (!rcu_pending(cpu))
>>  		return; /* if nothing for RCU to do. */
>> -	if (user ||
>> -	    (idle_cpu(cpu) && rcu_scheduler_active &&
>> -	     !in_softirq() && hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
>> +	if (user || rcu_idle_qs(cpu)) {
>>  
>>  		/*
>>  		 * Get here if this CPU took its interrupt from user
>>
> 
> 
> 
> 

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