linux-kernel - Re: [PATCH] RCU: don't turn off lockdep when find suspicious rcu_dereference

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Date:	Thu, 03 Jun 2010 17:22:04 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Daniel J Blueman <daniel.blueman@...il.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] RCU: don't turn off lockdep when find suspicious rcu_dereference_check()
 usage

Paul E. McKenney wrote:
> On Tue, Jun 01, 2010 at 02:06:13PM +0100, Daniel J Blueman wrote:
>> Hi Paul,
>>
>> With 2.6.35-rc1 and your patch in the context below, we still see
>> "include/linux/cgroup.h:534 invoked rcu_dereference_check() without
>> protection!", so need this additional patch:
>>
>> Acquire read-side RCU lock around task_group() calls, addressing
>> "include/linux/cgroup.h:534 invoked rcu_dereference_check() without
>> protection!" warning.
>>
>> Signed-off-by: Daniel J Blueman <daniel.blueman@...il.com>
> 
> Thank you, Daniel!  I have queued this for 2.6.35.
> 
> I had to apply the patch by hand due to line wrapping.  Could you please
> check your email-agent settings?  This simple patch was no problem to
> hand apply, but for a larger patch this process would be both tedious
> and error prone.
> 
> 							Thanx, Paul
> 
>> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
>> index 217e4a9..50ec9ea 100644
>> --- a/kernel/sched_fair.c
>> +++ b/kernel/sched_fair.c
>> @@ -1241,6 +1241,7 @@ static int wake_affine(struct sched_domain *sd,
>> struct task_struct *p, int sync)
>>  	 * effect of the currently running task from the load
>>  	 * of the current CPU:
>>  	 */
>> +	rcu_read_lock();
>>  	if (sync) {
>>  		tg = task_group(current);
>>  		weight = current->se.load.weight;
>> @@ -1250,6 +1251,7 @@ static int wake_affine(struct sched_domain *sd,
>> struct task_struct *p, int sync)
>>  	}
>>
>>  	tg = task_group(p);
>> +	rcu_read_unlock();

Hmmm.. I think it's not safe to access tg after rcu_read_unlock.

>>  	weight = p->se.load.weight;
>>
>>  	imbalance = 100 + (sd->imbalance_pct - 100) / 2;
>>
--
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