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Date: Tue, 14 Feb 2017 17:18:09 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Tony Lindgren <tony@atomide.com>, Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>, linux-pm@vger.kernel.org
Subject: Re: Regression in next with use printk_safe buffers in printk
Message-ID: <20170214161809.GQ6500@twins.programming.kicks-ass.net>
References: <20170213185956.GM3897@atomide.com>
 <20170214160140.GA401@tigerII.localdomain>
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On Wed, Feb 15, 2017 at 01:01:40AM +0900, Sergey Senozhatsky wrote:
> 
> but I'm a bit confused by rt_b->rt_runtime_lock in this unsafe lock
> scenario (so it's not ABBA, but ABAD)
> 
> >   lock(hrtimer_bases.lock);
> >                                lock(&rt_b->rt_runtime_lock);
> >                                lock(hrtimer_bases.lock);
> >   lock(tk_core);
> >
> >
> > Chain exists of:
> >
> >	tk_core --> &rt_b->rt_runtime_lock --> hrtimer_bases.lock
> 
> 
> I'm lacking some knowledge here, sorry. where does the tk_core --> &rt_b->rt_runtime_lock
> come from?

rt_b->rt_runtime_lock is one of the scheduler locks, since we do
printk() under tk_core, which does semaphore muck, which then includes
the entire scheduler chain of locks.