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

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Message-ID: <20180907145221.GE482@tigerII.localdomain>
Date:   Fri, 7 Sep 2018 23:52:21 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        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 (09/07/18 16:03), Peter Zijlstra wrote:
> > 
> > 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 ;-)

Talented, capable and tremendously clever people had spent decades on
making printk what it is today. I feel responsible for respecting that
effort and, thus, my vote would be to keep printk around for a while.
... we also support !CONFIG_PRINTK builds ;)

	-ss