linux-kernel - Re: [PATCH 1/3] printk: Release console_sem after logbuf

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Message-ID: <20110609202707.GA28951@elte.hu>
Date:	Thu, 9 Jun 2011 22:27:07 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, efault@....de,
	Arne Jansen <lists@...-jansens.de>
Subject: Re: [PATCH 1/3] printk: Release console_sem after logbuf_lock


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Thu, 09 Jun 2011 15:06:48 +0200
> Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> 
> > Release console_sem after unlocking the logbuf_lock so that we don't
> > generate wakeups while holding logbuf_lock. This avoids some lock
> > inversion troubles once we remove the lockdep_off bits between
> > logbuf_lock and rq->lock (prints while holding rq->lock vs doing
> > wakeups while holding logbuf_lock).
> > 
> > There's of course still an actual deadlock where the printk()s under
> > rq->lock will issue a wakeup from the up() call.
> > 
> > ...
> >
> > @@ -1271,8 +1273,8 @@ void console_unlock(void)
> >  	if (unlikely(exclusive_console))
> >  		exclusive_console = NULL;
> >  
> > -	up(&console_sem);
> >  	spin_unlock_irqrestore(&logbuf_lock, flags);
> > +	up(&console_sem);
> >  	if (wake_klogd)
> >  		wake_up_klogd();
> >  }
> 
> I have a horrible feeling that I put the up() inside logbuf_lock for
> Special And Cunning Reasons.  But I'm struggling to work out what they
> might have been and my archives only go back to October 2000(!).
> 
> Hate it when that happens.

Heh, here's what i told Peter two days ago when i saw that chunk:

  => Subject: printk: Release console_sem after logbuf_lock
  => i have some vague memories about some sort of complication in that area ...
  => but don't remember the specifics
  => only a 'there be dragons' mental marker

and i have to say that when i found a boot lockup during testing i 
was not surprised very much :)

Thanks,

	Ingo
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