linux-kernel - Re: lockdep warning after recent cleanup in console code

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Date:	Mon, 28 Apr 2014 21:24:16 +0200
From:	Jan Kara <jack@...e.cz>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jan Kara <jack@...e.cz>, Peter Zijlstra <peterz@...radead.org>,
	"Shevchenko, Andriy" <andriy.shevchenko@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: lockdep warning after recent cleanup in console code

On Mon 28-04-14 14:14:39, Steven Rostedt wrote:
> On Mon, 28 Apr 2014 19:51:39 +0200
> Jan Kara <jack@...e.cz> wrote:
> 
> > On Mon 28-04-14 13:43:31, Steven Rostedt wrote:
> > > Things have changed with regard to printk() in linux-next. Now it
> > > appears that lockdep is going haywire over it. I don't understand the
> > > exact reason for the lockdep_off() and lockdep_on() logic that is in
> > > printk(), but it obviously seems to be causing issues with the new
> > > changes.
> > > 
> > > Care to take a look?
> >   The obvious cause is that I moved lockdep_on() somewhat earlier in
> > vprintk_emit() so lockdep now covers more of printk code. And apparently
> > something is wrong there...
> > 
> 
> Exactly, and I rather know *exactly* what is wrong before we just start
> throwing patches at the problem and hope it goes away. That's not how
> to solve a software bug.
  So I had a look and we are missing mutex_release() in
console_trylock_for_printk() if we don't have a console to print to.
Attached patch should fix the problem.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR

View attachment "0001-printk-Fix-lockdep-instrumentation-of-console_sem.patch" of type "text/x-patch" (3384 bytes)