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Date:   Thu, 13 Feb 2020 13:48:59 -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: <20200213214859.GL2935@paulmck-ThinkPad-P72>
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On Thu, Feb 13, 2020 at 04:19:30PM -0500, Joel Fernandes wrote:
> On Thu, Feb 13, 2020 at 12:54:42PM -0800, Paul E. McKenney wrote:
> > 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()?
> 
> It seemed odd to me we were not allowing kprobe on the rcu_nmi_enter() but
> allowing it on exit (from a code reading standpoint) so my reaction was to
> add it to both, but we could probably keep that as a separate
> patch/discussion since it is slightly unrelated to the patch.. Sorry to
> confuse the topic.

Actually and perhaps unusually, I was not being sarcastic, but was instead
asking a serious question.  Is the current code correct?  Should the
current NOKPROBE_SYMBOL() be removed?  Should the other NOKPROBE_SYMBOL()
be added?  Something else?  And either way, why?

							Thanx, Paul

> thanks,
> 
>  - Joel
> 
> 
> > 							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();
> > >  }
> > >  
> > >  /*