linux-kernel - Re: [PATCH 01/20 -v5] printk - dont wakeup klogd with interrupts disabled

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Date:	Wed, 23 Jan 2008 12:27:12 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Daniel Walker <dwalker@...sta.com>
cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Bird <tim.bird@...sony.com>,
	Sam Ravnborg <sam@...nborg.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	John Stultz <johnstul@...ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 01/20 -v5] printk - dont wakeup klogd with interrupts
 disabled


On Wed, 23 Jan 2008, Daniel Walker wrote:

>
> On Wed, 2008-01-23 at 11:02 -0500, Steven Rostedt wrote:
>
> > +	if (!irqs_disabled() && wake_klogd)
> >  		wake_up_klogd();
>
> This causes a regression .. When printk is called during an OOPS in
> kernels without this change then the OOPS will get logged, since the
> logging process (klogd) is woken to handle the messages.. If you apply
> this change klogd doesn't wakeup, and hence doesn't log the oops.. So if
> you remove the wakeup here you have to add it someplace else to maintain
> the logging ..
>
> (I'm not theorizing here, I have defects logged against this specific
> piece of code..)

It wont get woken up anyway. Did you look at wake_up_klogd?

void wake_up_klogd(void)
{
	if (!oops_in_progress && waitqueue_active(&log_wait))
		wake_up_interruptible(&log_wait);
}


So if oops_in_progress is set, then it still wont get woken. Perhaps it
got woken some other way? Or is oops_in_progress not set in these oops?

One other solution is to make the runqueue locks visible externally. Like:

in sched.c:

int runqueue_is_locked(void)
{
	int cpu = get_cpu();
	struct rq *rq = cpu_rq(cpu);
	int ret;

	ret = spin_is_locked(&rq->lock);
	put_cpu();
	return ret;
}

And in printk we could do:

	if (wake_klogd && !runqueue_is_locked())
		wake_up_klogd();

This probably is the cleanest solution since it simply prevents the
deadlock from occurring.

-- Steve

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