linux-kernel - Re: [PATCH V2 2/7] rcu: cleanup rcu_preempt_deferred

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Date:   Sun, 3 Nov 2019 10:01:50 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Lai Jiangshan <laijs@...ux.alibaba.com>
Cc:     linux-kernel@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...nel.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 V2 2/7] rcu: cleanup rcu_preempt_deferred_qs()

Hi Jiangshan,


I haven't checked the correctness of this patch carefully, but..


On Sat, Nov 02, 2019 at 12:45:54PM +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.

rcu_preempt_deferred_qs_irqrestore() will report RCU qs, and may
eventually call swake_up() or its friends to wake up, say, the gp
kthread, and the wake up functions could go into the scheduler code
path which might have RCU read-side critical section in it, IOW,
accessing ->rcu_read_lock_nesting.

Again, haven't checked closely, but this argument in the commit log
seems untrue.

Regards,
Boqun

> 
> 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.
> 
> 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 aba5896d67e3..2fab8be2061f 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -552,16 +552,11 @@ 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
> 

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