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Date: Fri, 26 May 2017 09:47:00 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Masami Hiramatsu <mhiramat@kernel.org>
cc: LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
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        Sebastian Siewior <bigeasy@linutronix.de>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>
Subject: Re: [patch V3 25/32] kprobes: Cure hotplug lock ordering issues
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References: <20170524081511.203800767@linutronix.de> <20170524081549.104864779@linutronix.de> <20170525005410.f7142f05cd5d248600c61f96@kernel.org>
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On Thu, 25 May 2017, Masami Hiramatsu wrote:
> On Wed, 24 May 2017 10:15:36 +0200
> Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> > Converting the cpu hotplug locking to a percpu rwsem unearthed hidden lock
> > ordering problems.
> > 
> > There is a wide range of locks involved in this: kprobe_mutex,
> > jump_label_mutex, ftrace_lock, text_mutex, event_mutex,
> > func_hash->regex_lock and a gazillion of lock order permutations with
> > nested get_online_cpus() calls.
> 
> And module_mutex too ;-)

Indeed.

> > Some of those permutations are potential deadlocks even with the current
> > nesting hotplug locking scheme, but they can't be discovered by lockdep.
> > 
> > The conversion of the hotplug locking to a percpu rwsem requires to prevent
> > nested locking, so it's required to take the hotplug rwsem early in the
> > call chain and establish a proper lock order.
> > 
> > After quite some analysis and going down the wrong road severa times the
> > following lock order has been chosen:
> > 
> > kprobe_mutex -> cpus_rwsem -> jump_label_mutex -> text_mutex
> 
> This seems only change the locking order of module_mutex and
> cpus_rwsem. Previously module_mutex -> cpus_rwsem, now
> cpus_rwsem -> module_mutex. and it seems OK to me.
> (checked in module.c and other use cases of module_mutex)

This also changes the jump label / text mutex interaction with hotplug
locking if you look closely :)