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

[Wander Lairson Costa]

On Mon, Jan 23, 2023 at 1:30 PM Oleg Nesterov <oleg@xxxxxxxxxx> 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.