linux-kernel - Re: [PATCH v4 08/27] rcu/kprobes: Comment why rcu_nmi

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Message-ID: <20200226002755.GD9599@lenoir>
Date:   Wed, 26 Feb 2020 01:27:55 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        rostedt@...dmis.org, mingo@...nel.org, joel@...lfernandes.org,
        gregkh@...uxfoundation.org, gustavo@...eddedor.com,
        tglx@...utronix.de, paulmck@...nel.org, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        luto@...nel.org, tony.luck@...el.com, dan.carpenter@...cle.com,
        mhiramat@...nel.org
Subject: Re: [PATCH v4 08/27] rcu/kprobes: Comment why rcu_nmi_enter() is
 marked NOKPROBE

On Fri, Feb 21, 2020 at 02:34:24PM +0100, Peter Zijlstra wrote:
> From: Steven Rostedt (VMware) <rostedt@...dmis.org>
> 
> It's confusing that rcu_nmi_enter() is marked NOKPROBE and
> rcu_nmi_exit() is not. One may think that the exit needs to be marked
> for the same reason the enter is, as rcu_nmi_exit() reverts the RCU
> state back to what it was before rcu_nmi_enter(). But the reason has
> nothing to do with the state of RCU.
> 
> The breakpoint handler (int3 on x86) must not have any kprobe on it
> until the kprobe handler is called. Otherwise, it can cause an infinite
> recursion and crash the machine. It just so happens that
> rcu_nmi_enter() is called by the int3 handler before the kprobe handler
> can run, and therefore needs to be marked as NOKPROBE.
> 
> Comment this to remove the confusion to why rcu_nmi_enter() is marked
> NOKPROBE but rcu_nmi_exit() is not.
> 
> Reported-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
> Reviewed-by: Masami Hiramatsu <mhiramat@...nel.org>
> Acked-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> Link: https://lore.kernel.org/r/20200213163800.5c51a5f1@gandalf.local.home

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>