Date: Thu, 24 Jan 2008 11:36:54 +0100
From: Pavel Machek <pavel@ucw.cz>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Daniel Walker <dwalker@mvista.com>, LKML <linux-kernel@vger.kernel.org>,
       Ingo Molnar <mingo@elte.hu>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Christoph Hellwig <hch@infradead.org>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       Gregory Haskins <ghaskins@novell.com>,
       Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
       Thomas Gleixner <tglx@linutronix.de>, Tim Bird <tim.bird@am.sony.com>,
       Sam Ravnborg <sam@ravnborg.org>, "Frank Ch. Eigler" <fche@redhat.com>,
       Jan Kiszka <jan.kiszka@siemens.com>, John Stultz <johnstul@us.ibm.com>,
       Arjan van de Ven <arjan@infradead.org>,
       Steven Rostedt <srostedt@redhat.com>
Subject: Re: [PATCH 01/20 -v5] printk - dont wakeup klogd with interrupts
	disabled
Message-ID: <20080124103654.GB3433@elf.ucw.cz>
References: <20080123160236.969334052@goodmis.org> <20080123160441.236562634@goodmis.org> <1201107978.13596.16.camel@imap.mvista.com> <Pine.LNX.4.58.0801231213430.12932@gandalf.stny.rr.com>
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On Wed 2008-01-23 12:27:12, Steven Rostedt 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.

Yes please... this looks like a solution to me.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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