Re: [patch] Real-Time Preemption, -RT-2.6.12-rc1-V0.7.41-07

Herbert Xu
2005-03-23T09:39:35Z

Ingo Molnar <mingo@elte.hu> wrote:
>
> * Paul E. McKenney <paulmck@us.ibm.com> wrote:
>
>> +#ifdef CONFIG_PREEMPT_RCU
>> +
>> +void rcu_read_lock(void)
>> +{
>> + if (current->rcu_read_lock_nesting++ == 0) {
>> + current->rcu_data = &get_cpu_var(rcu_data);
>> + atomic_inc(&current->rcu_data->active_readers);
>> + put_cpu_var(rcu_data);
>>
>> Need an smp_mb() here for non-x86 CPUs. Otherwise, the CPU can
>> re-order parts of the critical section to precede the rcu_read_lock().
>> Could precede the put_cpu_var(), but why increase latency?
>
> ok. It's enough to put a barrier into the else branch here, because the
> atomic op in the main brain is a barrier by itself.

Since the else branch is only taken when rcu_read_lock_nesting > 0, do
we need the barrier at all?

Cheers,
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