linux-kernel - Re: [PATCH] printk/tracing: Do not trace printk_nmi

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Date:   Fri, 7 Sep 2018 16:03:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] printk/tracing: Do not trace printk_nmi_enter()

On Fri, Sep 07, 2018 at 10:01:28AM -0400, Steven Rostedt wrote:
> On Fri, 7 Sep 2018 09:55:33 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > On Fri, 7 Sep 2018 15:45:32 +0200
> > Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > Yes really, we should not muck with the IRQ state from NMI context.  
> > 
> > Right, and we didn't. Your patch didn't change anything, but allow for
> > printk_nmi_enter/exit() to be traced by ftrace, but that's wrong to
> > begin with because it ftrace_nmi_enter() hasn't been called yet.
> > 
> 
> I would even argue that placing printk_nmi_enter() between
> lockdep_off() and ftrace_nmi_enter() is wrong because if in the future
> printk_nmi_enter() were to do any ftrace tracing, it wont be caught, as
> it was by having it before lockdep_off().
> 
> printk_nmi_enter() should not muck with IRQ state, nor should it do any
> ftrace tracing. Since ftrace mucks with IRQ state when it gets enabled
> or disabled, it will screw up lockdep, and lockdep will complain. That
> way we can use lockdep not being off to catch this bug.

The very bestest solution is to rm -rf printk ;-)