linux-kernel - Re: 2.6.25-rc9 -- INFO: possible circular locking dependency detected

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Date:	Tue, 15 Apr 2008 19:22:10 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Miles Lane <miles.lane@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Ingo Molnar <mingo@...e.hu>
Subject: Re: 2.6.25-rc9 -- INFO: possible circular locking dependency
	detected

On Mon, Apr 14, 2008 at 09:35:39PM +0200, Heiko Carstens wrote:
> On Mon, Apr 14, 2008 at 09:16:42PM +0530, Gautham R Shenoy wrote:
> > On Mon, Apr 14, 2008 at 05:19:46PM +0200, Heiko Carstens wrote:
> > > On Mon, Apr 14, 2008 at 08:18:01PM +0530, Gautham R Shenoy wrote:
> > > > On Mon, Apr 14, 2008 at 02:42:27PM +0200, Peter Zijlstra wrote:
> > > 
> > > While you're fixing the cpu hotplug stuff anyway, there's still a bug
> > > present in a few modules init code:
> > > 
> > > Usually they do something like:
> > > 
> > > 	register_hotcpu_notifier(...);
> > > 	for_each_online_cpu(i)
> > > 		...
> > > 
> > > A module's init functions gets called from sys_init_module and there is nothing
> > > that would protect from cpu hotplug.
> > > Therefore the sequence of for_each_online_cpu() and register_hotcpu_notifier()
> > > better should be protected by a surrounding get/put_online_cpus() like this:
> > > 
> > > 	get_online_cpus();
> > > 	register_hotcpu_notifier(...);
> > > 	for_each_online_cpu(i)
> > > 		...
> > > 	put_online_cpus();
> > 
> > But shouldn't this be:
> > 	register_hotcpu_notifier(...);
> > 	get_online_cpus();
> > 	for_each_online_cpus()
> > 		...
> > 	put_online_cpus();
> > 
> > What's the problem with this ordering?
> 
> The problem here is that between register_hotcpu_notifier() and
> get_online_cpus() a cpu might have been hotplugged.
> So on cpu down the registered function might try to undo something that
> wasn't prepared in the first place.
> On cpu up however it will do things twice. Once for the cpus that got
> added between register_hotcpu_notifier() and for_each_online_cpus()
> and then again in the for_each_online_cpus() loop.
> 
> Of course all of these scenarios could be fixed in each driver, but that
> would be a lot of duplicated work. Making sure the combination of
> get_online_cpus() and register_hotcpu_notifier() cannot deadlock would
> make things much easier.

Ah, okay. Thanks for the explanation.
So how about having a new API,
something along the lines of:

kernel/cpu.c
------------------------------------------------------
register_hot_cpu_notifier_init(notifier_name, driver_hotcpu_init_function)
{
	mutex_lock(&cpu_add_remove_lock);
	get_online_cpus();
	__register_hot_cpu_notifier(notifier_name);
	driver_hotcpu_init_function();
	put_online_cpus();
	mutex_unlock(&cpu_add_remove_lock);
}

drivers/mydriver.c
--------------------------------------------------------------
driver_hotcpu_init_function()
{
	for_each_online_cpus()
		perform_subsystem_hotcpu_initialization();
}


driver_init()
{
	register_hotcpu_notifier_init(notifier_name,
			driver_hotcpu_init_function);
}



-- 
Thanks and Regards
gautham
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