linux-kernel - Re: [LKP] [sched] BUG: kernel boot hang

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Date:	Mon, 16 Feb 2015 19:22:38 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Huang Ying <ying.huang@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, LKP ML <lkp@...org>
Subject: Re: [LKP] [sched] BUG: kernel boot hang

On Mon, Feb 16, 2015 at 04:07:33PM +0100, Peter Zijlstra wrote:
> On Sun, Feb 15, 2015 at 01:18:07PM -0500, Steven Rostedt wrote:
> > On Sun, 15 Feb 2015 16:46:22 +0100
> > Frederic Weisbecker <fweisbec@...il.com> wrote:
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index c017a5f..a6d4d6c 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -2879,7 +2879,7 @@ void __sched schedule_preempt_disabled(void)
> > >  	preempt_disable();
> > >  }
> > >  
> > > -static void preempt_schedule_common(void)
> > > +static void __sched notrace preempt_schedule_common(void)
> > >  {
> > >  	do {
> > >  		preempt_count_add(PREEMPT_ACTIVE);
> > 
> > Ah, since I added better recursion protection code in function tracer
> > this didn't break that. But unfortunately, function graph tracer
> > doesn't have that protection.
> > 
> > If it traces between preempt_schedule() and where it sets
> > PREEMPT_ACTIVE, it can indeed go into an infinite recursion. Yeah,
> > preempt_schedule_common() should be notrace, at least until we change
> > function_graph to have that recursion protection.
> > 
> > Acked-by: Steven Rostedt <rostedt@...dmis.org>
> 
> 
> Can someone shoot me a proper patch with Changelog and such?

Sure, I just sent it as: "[PATCH] sched: Fix preempt_schedule_common() triggering tracing recursion"

Huang Ying hasn't yet reported a Tested-by but I could reproduce (with both function and function_graph tracers btw)
and the patch fixed it.

Thanks.
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