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

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Date:   Fri, 7 Sep 2018 10:28:34 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        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 2018-09-07 09:45:31, Peter Zijlstra wrote:
> On Thu, Sep 06, 2018 at 11:31:51AM +0900, Sergey Senozhatsky wrote:
> > An alternative option, thus, could be re-instating back the rule that
> > lockdep_off/on should be the first and the last thing we do in
> > nmi_enter/nmi_exit. E.g.
> > 
> > nmi_enter()
> > 	lockdep_off();
> > 	printk_nmi_enter();
> > 
> > nmi_exit()
> > 	printk_nmi_exit();
> > 	lockdep_on();
> 
> Yes that. Also, those should probably be inline functions.
> 
> ---
> Subject: locking/lockdep: Fix NMI handling
> 
> Someone put code in the NMI handler before lockdep_off(). Since lockdep
> is not NMI safe, this wrecks stuff.

My view is that nmi_enter() has to switch several features into
NMI-safe mode. The code must not trigger the other features when
they are still in the unsafe mode.

It is a chicken&egg problem. And it is hard to completely prevent
regressions caused by future changes.

I though that printk_nmi_enter() should never need any lockdep-related
code. On the other hand, people might want to printk debug messages
when lockdep_off() is called. This is why I put it in the current order.

That said, I am not against this change. Especially the inlining
is a good move. Note that lockdep_off()/lockdep_on() must not
be traced as well.

Best Regards,
Petr