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SubjectRe: [PATCH] RCU: don't turn off lockdep when find suspicious rcu_dereference_check() usage
> 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.

> Thanx, Paul
>
> Signed-off-by: Paul E. McKenney <paulmck@linux.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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