linux-kernel - Re: [PATCH 04/11] rcu: cleanup rcu_preempt_deferred

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Message-ID: <1b5cf860-3d4a-954c-09ac-6383b38da4cf@linux.alibaba.com>
Date:   Thu, 31 Oct 2019 22:35:22 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     paulmck@...nel.org
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: Re: [PATCH 04/11] rcu: cleanup rcu_preempt_deferred_qs()



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@...ux.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
>>