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Message-ID: <20080414154642.GA30018@in.ibm.com>
Date: Mon, 14 Apr 2008 21:16:42 +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 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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