Re: [PATCH 04/11] rcu: cleanup rcu_preempt_deferred_qs()

Lai Jiangshan
2019-10-31T14:35:33Z

On 2019/10/31 10:10 下午, Paul E. McKenney wrote:
> On Thu, Oct 31, 2019 at 10:07:59AM +0000, Lai Jiangshan wrote:
>> Don't need to set ->rcu_read_lock_nesting negative, irq-protected
>> rcu_preempt_deferred_qs_irqrestore() doesn't expect
>> ->rcu_read_lock_nesting to be negative to work, it even
>> doesn't access to ->rcu_read_lock_nesting any more.
>>
>> It is true that NMI over rcu_preempt_deferred_qs_irqrestore()
>> may access to ->rcu_read_lock_nesting, but it is still safe
>> since rcu_read_unlock_special() can protect itself from NMI.
>
> Hmmm... Testing identified the need for this one. But I will wait for
> your responses on the earlier patches before going any further through
> this series.

Hmmm... I was wrong, it should be after patch7 to avoid
the scheduler deadlock.

>
> Thanx, Paul
>
>> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
>> ---
>> kernel/rcu/tree_plugin.h | 5 -----
>> 1 file changed, 5 deletions(-)
>>
>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>> index 82595db04eec..9fe8138ed3c3 100644
>> --- a/kernel/rcu/tree_plugin.h
>> +++ b/kernel/rcu/tree_plugin.h
>> @@ -555,16 +555,11 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
>> static void rcu_preempt_deferred_qs(struct task_struct *t)
>> {
>> unsigned long flags;
>> - bool couldrecurse = t->rcu_read_lock_nesting >= 0;
>>
>> if (!rcu_preempt_need_deferred_qs(t))
>> return;
>> - if (couldrecurse)
>> - t->rcu_read_lock_nesting -= RCU_NEST_BIAS;
>> local_irq_save(flags);
>> rcu_preempt_deferred_qs_irqrestore(t, flags);
>> - if (couldrecurse)
>> - t->rcu_read_lock_nesting += RCU_NEST_BIAS;
>> }
>>
>> /*
>> --
>> 2.20.1
>>