Re: 2.6.25-rc9 -- INFO: possible circular locking dependencydetected

[Heiko Carstens]

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 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
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