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

[Wander Lairson Costa]

On Fri, Jan 27, 2023 at 12:55 PM Valentin Schneider <vschneid@xxxxxxxxxx> wrote:
>
> On 23/01/23 14:24, Wander Lairson Costa wrote:
> > 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.
>
> I'd tend to agree.
>
> > 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.
>
> My worry here is that it's easy to miss problematic callgraphs, and it's
> potentially easy for new ones to creep in. Having a solution within
> put_task_struct() itself would prevent that.
>

We could add a WARN_ON statement in put_task_struct() to detect such cases.

> Another thing, if you look at release_task_stack(), it either caches the
> outgoing stack for later use, or frees it via RCU (regardless of
> PREEMPT_RT). Perhaps we could follow that and just always punt the freeing
> of the task struct to RCU?
>

That's a point. Do you mean doing that even for !PREEMPT_RT?