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

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Date:   Fri, 7 Sep 2018 09:34:48 +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 Wed, Sep 05, 2018 at 09:33:34PM -0400, Steven Rostedt wrote:
>   do_idle {
> 
>     [interrupts enabled]
> 
>     <interrupt> [interrupts disabled]
> 	TRACE_IRQS_OFF [lockdep says irqs off]
> 	[...]
> 	TRACE_IRQS_IRET
> 	    test if pt_regs say return to interrupts enabled [yes]
> 	    TRACE_IRQS_ON [lockdep says irqs are on]
> 
> 	    <nmi>
> 		nmi_enter() {
> 		    printk_nmi_enter() [traced by ftrace]
> 		    [ hit ftrace breakpoint ]
> 		    <breakpoint exception>
> 			TRACE_IRQS_OFF [lockdep says irqs off]
> 			[...]
> 			TRACE_IRQS_IRET [return from breakpoint]
> 			   test if pt_regs say interrupts enabled [no]
> 			   [iret back to interrupt]
> 	   [iret back to code]
> 
>     tick_nohz_idle_enter() {
> 
> 	lockdep_assert_irqs_enabled() [lockdep say no!]

Isn't the problem that we muck with the IRQ state from NMI context? We
shouldn't be doing that.

The thing is, since we trace the IRQ state from within IRQ-disable,
since that's the only IRQ-safe option, it is very much not NMI-safe.

Your patch might avoid the symptom, but I don't think it cures the
fundamental problem.