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

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Message-ID: <20100604041047.GB2780@linux.vnet.ibm.com>
Date:	Thu, 3 Jun 2010 21:10:47 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Li Zefan <lizf@...fujitsu.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

On Fri, Jun 04, 2010 at 10:44:48AM +0800, Li Zefan wrote:
> > Seems worth reviewing the other uses of task_group():
> > 
> > 1.	set_task_rq() -- only a runqueue and a sched_rt_entity leave
> > 	the RCU read-side critical section.  Runqueues do persist.
> > 	I don't claim to understand the sched_rt_entity life cycle.
> > 
> > 2.	__sched_setscheduler() -- not clear to me that this one is
> > 	protected to begin with.  If it is somehow correctly protected,
> > 	it discards the RCU-protected pointer immediately, so is OK
> > 	otherwise.
> > 
> > 3.	cpu_cgroup_destroy() -- ditto.
> > 
> > 4.	cpu_shares_read_u64() -- ditto.
> > 
> > 5.	print_task() -- protected by rcu_read_lock() and discards the
> > 	RCU-protected pointer immediately, so this one is OK.
> > 
> > Any task_group() experts able to weigh in on #2, #3, and #4?
> > 
> 
> #3 and #4 are safe, because it's not calling task_group(), but
> cgroup_tg():
> 
> 	struct task_group *tg = cgroup_tg(cgrp);
> 
> As long as it's safe to access cgrp, it's safe to access tg.

Good point, thank you!

Any takers on #2?

							Thanx, Paul

> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > 
> > diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> > index 50ec9ea..224ef98 100644
> > --- a/kernel/sched_fair.c
> > +++ b/kernel/sched_fair.c
> > @@ -1251,7 +1251,6 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
> >  	}
> >  
> >  	tg = task_group(p);
> > -	rcu_read_unlock();
> >  	weight = p->se.load.weight;
> >  
> >  	imbalance = 100 + (sd->imbalance_pct - 100) / 2;
> > @@ -1268,6 +1267,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
> >  	balanced = !this_load ||
> >  		100*(this_load + effective_load(tg, this_cpu, weight, weight)) <=
> >  		imbalance*(load + effective_load(tg, prev_cpu, 0, weight));
> > +	rcu_read_unlock();
> >  
> 
> This is fine.
> 
> Another way is :
> 
> rcu_read_lock();
> tg = task_group(p);
> css_get(&tg->css);
> rcu_read_unlock();
> 
> /* do something */
> ...
> 
> css_put(&tg->css);
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