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Date:   Tue, 18 Feb 2020 12:18:06 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Masami Hiramatsu <mhiramat@kernel.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.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}()
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On Tue, Feb 18, 2020 at 12:46:09PM -0500, Steven Rostedt wrote:
> On Tue, 18 Feb 2020 13:33:35 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
> 
> > On Mon, 17 Feb 2020 08:31:12 -0800
> > "Paul E. McKenney" <paulmck@kernel.org> wrote:
> > >   
> > > > 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.  
> > 
> > Yes, git grep shows that arm64 is using rcu_nmi_enter() in
> > debug_exception_enter().
> > OK, let's keep it, but maybe it is good to update the comment for
> > arm64 too. What about following?
> > 
> > +/*
> > + * All functions in do_int3() on x86, do_debug_exception() on arm64 must be
> > + * marked NOKPROBE before kprobes handler is called.
> > + * ist_enter() on x86 and debug_exception_enter() on arm64 which is called
> > + * before kprobes handle happens to call rcu_nmi_enter() which means
> > + * that rcu_nmi_enter() must be marked NOKRPOBE.
> > + */
> > 
> 
> Ah, why don't we just say...
> 
> /*
>  * All functions called in the breakpoint trap handler (e.g. do_int3()
>  * on x86), must not allow kprobes until the kprobe breakpoint handler
>  * is called, otherwise it can cause an infinite recursion.
>  * On some archs, rcu_nmi_enter() is called in the breakpoint handler
>  * before the kprobe breakpoint handler is called, thus it must be
>  * marked as NOKPROBE.
>  */
> 
> And that way we don't make this an arch specific comment.

That looks good to me.  Masami, does this work for you?

							Thanx, Paul