linux-kernel - Re: [PATCH 1/2] module: use rcu to protect module list read

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Date:	Sun, 11 Mar 2012 18:53:52 +0800
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 1/2] module: use rcu to protect module list read

On 03/10/2012 11:25 PM, Eric Dumazet wrote:
> Le samedi 10 mars 2012 à 22:20 +0800, Cong Wang a écrit :
>> Now the read of module list is protected by preempt disable + *_rcu
>> list operations, this is odd, as RCU read lock should be able to
>> protect it directly. This patch makes the read of module list
>> protected by RCU read lock and the write still protected by
>> module_mutex.
>>
>
> Problem is that your patch does more than that.
>
> In set_all_modules_text_rw() and set_all_modules_text_ro() you removed
> the mutex in favor of rcu_read_lock()

Yeah, I was wrong, set_all_modules_text_rw() and 
set_all_modules_text_ro() mean to block module add/del, it is correct to 
use module_mutex.

>
> Also, module code uses synchronize_sched(), not synchronize_rcu()

Stupid me, I totally missed this, should use rcu_read_lock_sched()...

>
> Take a look at Documentation/RCU/whatisRCU.txt and see that
> preempt_disable() / preempt_enable() are documented as a right protect
> code, in line 333.
>
> You added races in /proc/modules as well.

Yeah, that list iteration is not protected by rcu yet.

>
> So I would say your patch is not needed at all : module code already
> uses RCU.
>
> What particular problem do you have with current code ?

preempt_disable() + list_*_rcu() looks a little weird for me, 
rcu_read_lock_sched() + list_*_rcu() is better, although they are 
functionally equal.

I will send out a new version.

Thanks for review!
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