linux-kernel - Re: Regression in next with use printk

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Date:   Tue, 14 Feb 2017 17:18:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Tony Lindgren <tony@...mide.com>, Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>, linux-pm@...r.kernel.org
Subject: Re: Regression in next with use printk_safe buffers in printk

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.