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

Wander Lairson Costa
2023-01-23T17:25:45Z

On Mon, Jan 23, 2023 at 1:30 PM Oleg Nesterov <oleg@redhat.com> wrote:
>
> 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);
> }
> }
>
> ?

Yeah, that was one approach I thought about. I chose to use an
explicit function because I assumed calling __put_task_struct() from a
non-preemptable context should be the exception, not the rule.
Therefore (if I am correct in my assumption), it would make sense for
only some call sites to pay the overhead price for it. But this is
just a guess, and I have no evidence to support my claim.