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


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