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

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Date:   Thu, 6 Sep 2018 11:22:33 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.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 Thu, 6 Sep 2018 11:31:51 +0900
Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> wrote:

> Hello,
> 
> On (09/05/18 21:33), 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!]
> > 
> > Although interrupts are indeed enabled, lockdep thinks it is not, and since
> > we now do asserts via lockdep, it gives a false warning. The issue here is
> > that printk_nmi_enter() is called before lockdep_off(), which disables
> > lockdep (for this reason) in NMIs. By simply not allowing ftrace to see
> > printk_nmi_enter() (via notrace annotation) we keep lockdep from getting
> > confused.  
> 
> Great catch and I like the patch!
> 
> Indeed, with printk_nmi we changed the "everything that nmi_enter does
> should happen after lockdep_off" to "everything that nmi_enter does should
> happen after printk_nmi_enter" // +similar change to nmi_exit.
> 
> 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();
> 
> I guess that we can keep printk_nmi_enter/printk_nmi_exit at the top
> and at the bottom of nmi_enter/nmi_exit correspondingly just in case
> if lockdep_off/lockdep_on sometime in the future starts invoking printk(),
> which would deadlock us. Hence

I thought about doing the move first, but figured it wasn't needed, as
Petr said, it's before ftrace_nmi_enter() and should be notrace
regrardless.

> 
> Acked-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> 
> 
> > Cc: stable@...r.kernel.org
> > Fixes: 099f1c84c0052 ("printk: introduce per-cpu safe_print seq buffer")  
> 
> A minor nitpick,
> 
> printk_nmi was introduced by 42a0bb3f71383b457 ("printk/nmi: generic
> solution for safe printk in NMI") - in that commit we declared a new
> printk_nmi_enter/exit should be the first and the last thing we do in
> nmi_enter/nmi_exit rule.

Thanks I'll update the change log. Grumble, my git blame foo is getting
rusty.

-- Steve