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

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Date:	Tue, 13 Mar 2012 18:09:54 +0800
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] module: use rcu to protect module list read

On 03/13/2012 08:37 AM, Rusty Russell wrote:
> On Sat, 10 Mar 2012 07:25:57 -0800, Eric Dumazet<eric.dumazet@...il.com>  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()
>>
>> Also, module code uses synchronize_sched(), not synchronize_rcu()
>
> Yes, but only for paranoia.  Really, it's vs. stop_machine().
>
>> 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.
>
> I'm surprised that patch didn't warn... CONFIG_DEBUG_ATOMIC_SLEEP=y
> might help here....

Ok I will enable it for testing.

Thanks.
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