Re: [PATCH v2 1/4] sched/task: Add the put_task_struct_atomic_safe function

Oleg Nesterov
2023-01-23T16:31:19Z

On 01/20, Wander Lairson Costa wrote:
>
> +static inline void put_task_struct_atomic_safe(struct task_struct *task)
> +{
> + if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> + /*
> + * Decrement the refcount explicitly to avoid unnecessarily
> + * calling call_rcu.
> + */
> + if (refcount_dec_and_test(&task->usage))
> + /*
> + * under PREEMPT_RT, we can't call put_task_struct
> + * in atomic context because it will indirectly
> + * acquire sleeping locks.
> + */
> + call_rcu(&task->rcu, __delayed_put_task_struct);
^^^^^^^^^
I am not sure the usage of task->rcu is safe...

Suppose that, before __delayed_put_task_struct() is called by RCU, this task
does the last schedule and calls put_task_struct_rcu_user().

And, can't we simply turn put_task_struct() into something like

put_task_struct(struct task_struct *t)
{
if (refcount_dec_and_test(&t->usage)) {
if (IS_ENABLED(CONFIG_PREEMPT_RT)
&& (in_atomic() || irqs_disabled()))
call_rcu(...);
else
__put_task_struct(t);
}
}

?

Oleg.