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Date:   Tue, 18 Feb 2020 12:46:09 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     paulmck@kernel.org, Joel Fernandes <joel@joelfernandes.org>,
        Peter Zijlstra <peterz@infradead.org>,
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Subject: Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}()
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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.

-- Steve