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

[Gautham R Shenoy <ego@in.ibm.com>]

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?

> 
> But as far as I can see that can lead to a deadlock if e.g. cpu 0 would
> execute the code above whild cpu 1 is executing some cpu hotplug code:
> 
> cpu0: get_online_cpus()
>        -> increase cpu_hotplug.refcount
> cpu1: cpu_down()
>        -> cpu_maps_update_begin()
>        -> grab cpu_add_remove_lock
>        -> wait for cpu_hotplug.refcount to drop to zero again
> cpu0: register_hotcpu_notifier()
>        -> cpu_maps_update_begin
>        -> tries to grab cpu_add_remove_lock that cpu 1 holds already
>           -> dead

-- 
Thanks and Regards
gautham