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Date: Fri, 31 Jul 2015 19:14:57 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-kernel@vger.kernel.org, Rusty Russell <rusty@rustcorp.com.au>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
        tglx@linutronix.de
Subject: Re: [RFC 1/4] module: use rcu_read_lock() while walking over a RCU
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On Fri, Jul 31, 2015 at 07:08:05PM +0200, Sebastian Andrzej Siewior wrote:
> The `modules' list uses list_for_each_entry_rcu() itarator.
> rcu_readlock() protects here against against module removal invoked from
> another CPU while preempt_disable() does not.

Not so, it does in fact.

> I don't understand what syncs in CONFIG_MODULES_TREE_LOOKUP against
> module removal. In the other (RCU) case there is synchronize_sched().

Its using rcu-sched aka preempt_disable.

if you want to use rcu primitives, use rcu_read_lock_sched(), but that
is in fact identical to preempt_disable().

Your patch breaks things.