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Date:   Tue, 18 Feb 2020 08:12:31 -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>,
        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 01:33:35PM +0900, Masami Hiramatsu 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.
> + */

Would it work to describe the general problem, then give x86 details
as a specific example, as follows?

/*
 * On some architectures, certain exceptions prohibit use of kprobes until
 * the exception code path reaches a certain point.  For example, on x86 all
 * functions called by do_int3() must be marked NOKPROBE.  However, once
 * kprobe_int3_handler() is called, kprobing is permitted.  Specifically,
 * ist_enter() is called in do_int3() before kprobe_int3_handle().
 * Furthermore, ist_enter() calls rcu_nmi_enter(), which means that
 * rcu_nmi_enter() must be marked NOKRPOBE.
 */

That way, I don't feel like I need to update the commment each time
a new architecture adds this capability.  ;-)

							Thanx, Paul