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

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Message-ID: <20140429210750.GE29634@quack.suse.cz>
Date:	Tue, 29 Apr 2014 23:07:50 +0200
From:	Jan Kara <jack@...e.cz>
To:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:	Jan Kara <jack@...e.cz>, Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	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 Tue 29-04-14 11:38:04, Andy Shevchenko wrote:
> On Mon, 2014-04-28 at 21:24 +0200, Jan Kara wrote:
> > 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.
> 
> Besides it doesn't apply clearly on top of today's linux-next, it
> doesn't fix the issue, but modifies it a bit.
  Sorry, I was too tired and missed conversion of one place. Attached is a
new version of the patch which also applies cleanly against linux-next.

								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" (3959 bytes)