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Date:   Thu, 13 Feb 2020 12:54:42 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        rostedt@goodmis.org, mingo@kernel.org, gregkh@linuxfoundation.org,
        gustavo@embeddedor.com, tglx@linutronix.de, josh@joshtriplett.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com
Subject: Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}()
Message-ID: <20200213205442.GK2935@paulmck-ThinkPad-P72>
Reply-To: paulmck@kernel.org
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On Thu, Feb 13, 2020 at 03:44:44PM -0500, Joel Fernandes wrote:
> On Thu, Feb 13, 2020 at 10:56:12AM -0800, Paul E. McKenney wrote:
> [...] 
> > > > It might well be that I could make these functions be NMI-safe, but
> > > > rcu_prepare_for_idle() in particular would be a bit ugly at best.
> > > > So, before looking into that, I have a question.  Given these proposed
> > > > changes, will rcu_nmi_exit_common() and rcu_nmi_enter_common() be able
> > > > to just use in_nmi()?
> > > 
> > > That _should_ already be the case today. That is, if we end up in a
> > > tracer and in_nmi() is unreliable we're already screwed anyway.
> > 
> > So something like this, then?  This is untested, probably doesn't even
> > build, and could use some careful review from both Peter and Steve,
> > at least.  As in the below is the second version of the patch, the first
> > having been missing a couple of important "!" characters.
> 
> I removed the static from rcu_nmi_enter()/exit() as it is called from
> outside, that makes it build now. Updated below is Paul's diff. I also added
> NOKPROBE_SYMBOL() to rcu_nmi_exit() to match rcu_nmi_enter() since it seemed
> asymmetric.

My compiler complained about the static and the __always_inline, so I
fixed those.  But please help me out on adding the NOKPROBE_SYMBOL()
to rcu_nmi_exit().  What bad thing happens if we leave this on only
rcu_nmi_enter()?

							Thanx, Paul

> ---8<-----------------------
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index d91c9156fab2e..bbcc7767f18ee 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -614,16 +614,18 @@ void rcu_user_enter(void)
>  }
>  #endif /* CONFIG_NO_HZ_FULL */
>  
> -/*
> +/**
> + * rcu_nmi_exit - inform RCU of exit from NMI context
> + *
>   * If we are returning from the outermost NMI handler that interrupted an
>   * RCU-idle period, update rdp->dynticks and rdp->dynticks_nmi_nesting
>   * to let the RCU grace-period handling know that the CPU is back to
>   * being RCU-idle.
>   *
> - * If you add or remove a call to rcu_nmi_exit_common(), be sure to test
> + * If you add or remove a call to rcu_nmi_exit(), be sure to test
>   * with CONFIG_RCU_EQS_DEBUG=y.
>   */
> -static __always_inline void rcu_nmi_exit_common(bool irq)
> +__always_inline void rcu_nmi_exit(void)
>  {
>  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
>  
> @@ -651,25 +653,15 @@ static __always_inline void rcu_nmi_exit_common(bool irq)
>  	trace_rcu_dyntick(TPS("Startirq"), rdp->dynticks_nmi_nesting, 0, atomic_read(&rdp->dynticks));
>  	WRITE_ONCE(rdp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
>  
> -	if (irq)
> +	if (!in_nmi())
>  		rcu_prepare_for_idle();
>  
>  	rcu_dynticks_eqs_enter();
>  
> -	if (irq)
> +	if (!in_nmi())
>  		rcu_dynticks_task_enter();
>  }
> -
> -/**
> - * rcu_nmi_exit - inform RCU of exit from NMI context
> - *
> - * If you add or remove a call to rcu_nmi_exit(), be sure to test
> - * with CONFIG_RCU_EQS_DEBUG=y.
> - */
> -void rcu_nmi_exit(void)
> -{
> -	rcu_nmi_exit_common(false);
> -}
> +NOKPROBE_SYMBOL(rcu_nmi_exit);
>  
>  /**
>   * rcu_irq_exit - inform RCU that current CPU is exiting irq towards idle
> @@ -693,7 +685,7 @@ void rcu_nmi_exit(void)
>  void rcu_irq_exit(void)
>  {
>  	lockdep_assert_irqs_disabled();
> -	rcu_nmi_exit_common(true);
> +	rcu_nmi_exit();
>  }
>  
>  /*
> @@ -777,7 +769,7 @@ void rcu_user_exit(void)
>  #endif /* CONFIG_NO_HZ_FULL */
>  
>  /**
> - * rcu_nmi_enter_common - inform RCU of entry to NMI context
> + * rcu_nmi_enter - inform RCU of entry to NMI context
>   * @irq: Is this call from rcu_irq_enter?
>   *
>   * If the CPU was idle from RCU's viewpoint, update rdp->dynticks and
> @@ -786,10 +778,10 @@ void rcu_user_exit(void)
>   * long as the nesting level does not overflow an int.  (You will probably
>   * run out of stack space first.)
>   *
> - * If you add or remove a call to rcu_nmi_enter_common(), be sure to test
> + * If you add or remove a call to rcu_nmi_enter(), be sure to test
>   * with CONFIG_RCU_EQS_DEBUG=y.
>   */
> -static __always_inline void rcu_nmi_enter_common(bool irq)
> +__always_inline void rcu_nmi_enter(void)
>  {
>  	long incby = 2;
>  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> @@ -807,12 +799,12 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
>  	 */
>  	if (rcu_dynticks_curr_cpu_in_eqs()) {
>  
> -		if (irq)
> +		if (!in_nmi())
>  			rcu_dynticks_task_exit();
>  
>  		rcu_dynticks_eqs_exit();
>  
> -		if (irq)
> +		if (!in_nmi())
>  			rcu_cleanup_after_idle();
>  
>  		incby = 1;
> @@ -834,14 +826,6 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
>  		   rdp->dynticks_nmi_nesting + incby);
>  	barrier();
>  }
> -
> -/**
> - * rcu_nmi_enter - inform RCU of entry to NMI context
> - */
> -void rcu_nmi_enter(void)
> -{
> -	rcu_nmi_enter_common(false);
> -}
>  NOKPROBE_SYMBOL(rcu_nmi_enter);
>  
>  /**
> @@ -869,7 +853,7 @@ NOKPROBE_SYMBOL(rcu_nmi_enter);
>  void rcu_irq_enter(void)
>  {
>  	lockdep_assert_irqs_disabled();
> -	rcu_nmi_enter_common(true);
> +	rcu_nmi_enter();
>  }
>  
>  /*