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Date:   Fri, 14 Feb 2020 15:19:06 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     paulmck@kernel.org
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Peter Zijlstra <peterz@infradead.org>,
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        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
        Masami Hiramatsu <mhiramat@kernel.org>
Subject: Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}()
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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!

-- 
Masami Hiramatsu <mhiramat@kernel.org>