Re: [debug patch] printk: Add a printk killswitch to robustify NMIwatchdog messages

[Arne Jansen]

On 06.06.2011 20:07, Ingo Molnar wrote:
> * Arne Jansen<lists@xxxxxxxxxxxxxx>  wrote:
>
> > On 06.06.2011 19:11, Ingo Molnar wrote:
> > > * Peter Zijlstra<peterz@xxxxxxxxxxxxx>   wrote:
> >
> > > > +void printk_tick(void)
> > > > +{
> > > > +	if (!__this_cpu_read(printk_pending))
> > > > +		return;
> > > > +
> > > > +	/*
> > > > +	 * Try to acquire and then immediately release the
> > > > +	 * console semaphore. The release will do all the
> > > > +	 * actual magic (print out buffers, wake up klogd,
> > > > +	 * etc).
> > > > +	 */
> > > > +	if (console_trylock_for_printk(smp_processor_id())) {
> > > > +		console_unlock();
> > > > +		__this_cpu_write(printk_pending, 0);
> > > > +	}
> > > > +}
> > >
> > > Arne does this fix the hang you are seeing?
> >
> > What do you want me to test? just replace printk_tick with the
> > above version? If I do that, the machine doesn't even boot up any
> > more.
>
> Yeah.
>
> So i think we want two patches:
>
>   - The first one that minimally removes the lockdep_off()/on() dance
>     and fixes the regression: the patch that i sent earlier today.
>     I *think* that should fix the crash.

Isn't the regression just the false lockdep_assert_held(&p->pi_lock)?
The patch Peter sent earlier seems like the minimal changeset to fix
that, plus it fixes a bug that might pop up somewhere else, too.

>     3.0 material.
>
>   - The second one that moves console_sem wakeups to the jiffies tick.
>     It does not push the acquiring and the console->write() calls to
>     jiffies context, only delays the wakeup.
>
>     3.1 material.
>
> Thanks,
>
> 	Ingo

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