linux-kernel - Re: [PATCH] printk: robustify printk

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Date:	Fri, 08 Aug 2008 21:21:08 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	torvalds@...ux-foundation.org, mingo@...e.hu, tglx@...utronix.de,
	marcin.slusarz@...il.com, linux-kernel@...r.kernel.org,
	davem@...emloft.net, rostedt@...dmis.org,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH] printk: robustify printk

On Fri, 2008-08-08 at 12:14 -0700, Andrew Morton wrote:
> On Fri, 08 Aug 2008 20:14:28 +0200
> Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> 
> >  void wake_up_klogd(void)
> >  {
> > -	if (!oops_in_progress && waitqueue_active(&log_wait))
> > -		wake_up_interruptible(&log_wait);
> > +	unsigned long flags;
> > +	struct klogd_wakeup_state *kws;
> > +
> > +	if (!waitqueue_active(&log_wait))
> > +		return;
> > +
> > +	local_irq_save(flags);
> > +	kws = &__get_cpu_var(kws);
> > +	if (!kws->pending) {
> > +		kws->pending = 1;
> > +		call_rcu(&kws->head, __wake_up_klogd);
> > +	}
> > +	local_irq_restore(flags);
> >  }
> 
> Note that kernel/rcupreempt.c's flavour of call_rcu() takes
> RCU_DATA_ME().lock, so there are still code sites from which a printk
> can deadlock.  Only now, it is config-dependent.
> 
> From a quick look it appears that large amounts of kernel/rcupreempt.c
> are now a printk-free zone.

Drad, missed that bit, I did look at the calling end, but forgot the
call_rcu() end :-/

The initial printk_tick() based implementation didn't suffer this
problem, should we revert to that scheme?

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