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

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Date:   Fri, 1 Nov 2019 02:33:19 +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 11:07 下午, Paul E. McKenney wrote:
> On Thu, Oct 31, 2019 at 10:35:22PM +0800, Lai Jiangshan wrote:
>> 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.
> 
> I was wondering about that.  ;-)
> 

This patch was split from the core patch(patch8: don't use negative 
->rcu_read_lock_nesting).

When I reordered "fixing something" as patch1/2, I reordered
it close to the patch of clean up rcu_preempt_deferred_qs_irqrestore
caused this mistake.

I will reorder it back later and "fixing something" is fixing
nothing and I will drop patch 1/2. Could you continue to review
further through this series please? Sorry for any mistakes.

Thanks
Lai

> 							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
>>>>