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

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Date:	Wed, 23 Jan 2008 12:32:49 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Daniel Walker <dwalker@...sta.com>,
	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>,
	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

* Steven Rostedt (rostedt@...dmis.org) wrote:
> 
> 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.
> 


FYI :

kernel/panic.c

NORET_TYPE void panic(const char * fmt, ...)
{
....
bust_spinlocks(1);
...
bust_spinlocks(0);
...

In bust_spinlocks :


void __attribute__((weak)) bust_spinlocks(int yes)
{
        if (yes) {
                ++oops_in_progress;
        } else {
#ifdef CONFIG_VT
                unblank_screen();
#endif
                if (--oops_in_progress == 0)
                        wake_up_klogd();
        }
}

Where the final wake_up happens while oops_in_progress is 0, but
interrupts are still disabled.

So about my previous email, proposing testing oops_in_progress, it just
won't work.

Mathieu

> -- Steve
> 

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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