Re: [PATCH 1/3] printk: Release console_sem after logbuf_lock

Ingo Molnar
2011-06-09T20:27:17Z

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

> On Thu, 09 Jun 2011 15:06:48 +0200
> Peter Zijlstra <a.p.zijlstra@chello.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