linux-kernel - Re: [RFC 1/4] module: use rcu_read_lock() while walking over a RCU protected list

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Message-ID: <20150731171457.GZ25159@twins.programming.kicks-ass.net>
Date:	Fri, 31 Jul 2015 19:14:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	tglx@...utronix.de
Subject: Re: [RFC 1/4] module: use rcu_read_lock() while walking over a RCU
 protected list

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.
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