linux-kernel - Re: [xen] double fault: 0000 [#1] PREEMPT SMP DEBUG

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Date:	Wed, 9 Oct 2013 08:46:02 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	xen-devel@...ts.xenproject.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [xen] double fault: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC

On Tue, Oct 08, 2013 at 03:48:40PM -0700, Greg KH wrote:
> On Tue, Oct 08, 2013 at 11:14:17PM +0100, Russell King - ARM Linux wrote:
> > On Tue, Oct 08, 2013 at 08:17:42PM +0800, Fengguang Wu wrote:
> > > I find the above debug messages very helpful in locating the buggy
> > > driver. How about enabling it whenever CONFIG_DEBUG_KOBJECT_RELEASE is
> > > enabled? Something like
> > > 
> > >  #ifdef CONFIG_DEBUG_KOBJECT_RELEASE
> > > -       pr_debug("kobject: '%s' (%p): %s, parent %p (delayed)\n",
> > > +       printk(KERN_INFO "kobject: '%s' (%p): %s, parent %p (delayed)\n",
> > >                  kobject_name(kobj), kobj, __func__, kobj->parent);
> > > 
> > > pr_debug() won't be displayed by default, and it depends on
> > > CONFIG_DYNAMIC_DEBUG.
> > 
> > Please consider using pr_info() instead of printk(KERN_INFO - it's
> > slightly less typing.

Good suggestion!

> > I can see that being a useful change while we have these problematical
> > instances to track down, but I do wonder whether it'll make the thing
> > too noisy.  Does anyone have other opinions on this point?  Linus?
> > Greg?
> 
> It's going to make things really noisy at boot time, but then it should
> settle down and not be bad at all.

FYI, in the randconfig kernel involved in this thread, it will emit
about 20 lines of dmesg.

> Let's try it and see if it helps or not.

OK, I'll submit a patch. These messages would allow me to do a
statistic analyze of similar bugs: since I'm testing 100+ new
randconfigs every day, some of them will include the buggy drivers and
some not, we can collect all these lines

[    2.929669] kobject: 'drm' (ffff880006db2848): kobject_release, parent ffff880000189648 (delayed)
...
[    3.750200] kobject: 'mc13783-adc' (ffff880007707000): kobject_release, parent ffff880000189248 (delayed)

, count how many times each one shows up in the GOOD kernel boots and
how many show up in the BAD boots. Then we may be able to learn which
drivers are likely buggy and should be manually checked.

Thanks,
Fengguang
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