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Date:   Mon, 17 Feb 2020 08:31:12 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Peter Zijlstra <peterz@infradead.org>,
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        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}()
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On Mon, Feb 17, 2020 at 05:55:19PM +0900, Masami Hiramatsu wrote:
> On Sat, 15 Feb 2020 06:59:34 -0800
> "Paul E. McKenney" <paulmck@kernel.org> wrote:
> 
> > On Fri, Feb 14, 2020 at 03:19:06PM +0900, Masami Hiramatsu wrote:
> > > On Thu, 13 Feb 2020 14:39:18 -0800
> > > "Paul E. McKenney" <paulmck@kernel.org> wrote:
> > > 
> > > > On Thu, Feb 13, 2020 at 05:04:51PM -0500, Steven Rostedt wrote:
> > > > > On Thu, 13 Feb 2020 13:50:04 -0800
> > > > > "Paul E. McKenney" <paulmck@kernel.org> wrote:
> > > > > 
> > > > > > On Thu, Feb 13, 2020 at 04:38:25PM -0500, Steven Rostedt wrote:
> > > > > > > [ Added Masami ]
> > > > > > > 
> > > > > > > On Thu, 13 Feb 2020 16:19:30 -0500
> > > > > > > Joel Fernandes <joel@joelfernandes.org> 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.
> > > > > > > >  
> > > > > > > 
> > > > > > > rcu_nmi_enter() was marked NOKPROBE or other reasons. See commit
> > > > > > > c13324a505c77 ("x86/kprobes: Prohibit probing on functions before
> > > > > > > kprobe_int3_handler()")
> > > > > > > 
> > > > > > > The issue was that we must not allow anything in do_int3() call kprobe
> > > > > > > code before kprobe_int3_handler() is called. Because ist_enter() (in
> > > > > > > do_int3()) calls rcu_nmi_enter() it had to be marked NOKPROBE. It had
> > > > > > > nothing to do with it being RCU nor NMI, but because it was simply
> > > > > > > called in do_int3().
> > > > > > > 
> > > > > > > Thus, there's no reason to make rcu_nmi_exit() NOKPROBE. But a commont
> > > > > > > to why rcu_nmi_enter() would probably be useful, like below:  
> > > > > > 
> > > > > > Thank you, Steve!  Could I please have your Signed-off-by for this?
> > > > > 
> > > > > Sure, but it was untested ;-)
> > > > 
> > > > No problem!  I will fire up rcutorture on it.  ;-)
> > > > 
> > > > But experience indicates that you cannot even make a joke around here.
> > > > There is probably already someone out there somewhere building a
> > > > comment-checker based on deep semantic analysis and machine learning.  :-/
> > > > 
> > > > > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> > > > > 
> > > > > I'd like a Reviewed-by from Masami though.
> > > > 
> > > > Sounds good!  Masami, would you be willing to review?
> > > 
> > > Yes, the functions before calling kprobe_int3_handler() must not
> > > be kprobed. It can cause an infinite recursive int3 trapping.
> > > 
> > > Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
> > 
> > Thank you both!
> > 
> > Like this?
> > 
> > 							Thanx, Paul
> > 
> 
> This is good to me.

Thank you for looking it over!  (I already have your

> BTW, if you consider the x86 specific code is in the generic file,
> we can move NOKPROBE_SYMBOL() in arch/x86/kernel/traps.c.
> (Sorry, I've hit this idea right now)

Might this affect other architectures with NMIs and probe-like things?
If so, it might make sense to leave it where it is.

							Thanx, Paul

> Thank you,
> 
> > ------------------------------------------------------------------------
> > 
> > commit 1817fdc8f4e4bd18c76305c9b937fb0dccbb1583
> > Author: Steven Rostedt <rostedt@goodmis.org>
> > Date:   Sat Feb 15 06:54:50 2020 -0800
> > 
> >     rcu: Provide comment for NOKPROBE() on rcu_nmi_enter()
> >     
> >     The rcu_nmi_enter() function was marked NOKPROBE() by commit
> >     c13324a505c77 ("x86/kprobes: Prohibit probing on functions before
> >     kprobe_int3_handler()") because the do_int3() call kprobe code must
> >     not be invoked before kprobe_int3_handler() is called.  It turns out
> >     that ist_enter() (in do_int3()) calls rcu_nmi_enter(), hence the
> >     marking NOKPROBE() being added to rcu_nmi_enter().
> >     
> >     This commit therefore adds a comment documenting this line of reasoning.
> >     
> >     Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> >     Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
> >     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 132b53e..4a885af 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -835,6 +835,12 @@ void rcu_nmi_enter(void)
> >  		   rdp->dynticks_nmi_nesting + incby);
> >  	barrier();
> >  }
> > +/*
> > + * On x86, All functions in do_int3() must be marked NOKPROBE before
> > + * kprobe_int3_handler() is called. ist_enter() which is called in do_int3()
> > + * before kprobe_int3_handle() happens to call rcu_nmi_enter() which means
> > + * that rcu_nmi_enter() must be marked NOKRPOBE.
> > + */
> >  NOKPROBE_SYMBOL(rcu_nmi_enter);
> >  
> >  /**
> 
> 
> -- 
> Masami Hiramatsu <mhiramat@kernel.org>