linux-kernel - Re: [PATCH 2/3] livepatch/rcu: Warn when system consistency is broken in RCU code

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Message-ID: <20170508171642.7968c110@gandalf.local.home>
Date:   Mon, 8 May 2017 17:16:42 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Petr Mladek <pmladek@...e.com>, Jessica Yu <jeyu@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] livepatch/rcu: Warn when system consistency is
 broken in RCU code

On Mon, 8 May 2017 15:43:33 -0500
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> Ok, so just to clarify.  Is there a bug in the ftrace stack tracer in
> the following situation?
> 
> 1. RCU isn't watching
> 2. An NMI hits
> 3. ist_enter() calls into the ftrace stack tracer, before
>    rcu_nmi_enter() is called, so RCU isn't watching yet

No, because in the ftrace stack tracer there's:

	if (in_nmi())
		return;

At the very beginning.

-- Steve

> 4. The ftrace stack tracer calls rcu_irq_enter(), which has no effect,
>    so RCU still isn't watching
> 5. Hilarity ensues in the ftrace stack tracer
>