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    SubjectRe: [PATCH v7 2/3] sched/task: Add the put_task_struct_atomic_safe() function
    On Thu, May 04, 2023 at 10:32:31AM +0100, Valentin Schneider wrote:
    > On 04/05/23 10:42, Peter Zijlstra wrote:
    > > On Tue, Apr 25, 2023 at 08:43:02AM -0300, Wander Lairson Costa wrote:
    > >> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
    > >> index b597b97b1f8f..cf774b83b2ec 100644
    > >> --- a/include/linux/sched/task.h
    > >> +++ b/include/linux/sched/task.h
    > >> @@ -141,6 +141,41 @@ static inline void put_task_struct_many(struct task_struct *t, int nr)
    > >>
    > >> void put_task_struct_rcu_user(struct task_struct *task);
    > >>
    > >> +extern void __delayed_put_task_struct(struct rcu_head *rhp);
    > >> +
    > >> +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() will schedule __delayed_put_task_struct()
    > >> + * to be called in process context.
    > >> + *
    > >> + * __put_task_struct() is called when
    > >> + * refcount_dec_and_test(&t->usage) succeeds.
    > >> + *
    > >> + * This means that it can't conflict with
    > >> + * put_task_struct_rcu_user() which abuses ->rcu the same
    > >> + * way; rcu_users has a reference so task->usage can't be
    > >> + * zero after rcu_users 1 -> 0 transition.
    > >> + *
    > >> + * delayed_free_task() also uses ->rcu, but it is only called
    > >> + * when it fails to fork a process. Therefore, there is no
    > >> + * way it can conflict with put_task_struct().
    > >> + */
    > >> + call_rcu(&task->rcu, __delayed_put_task_struct);
    > >> + } else {
    > >> + put_task_struct(task);
    > >> + }
    > >> +}
    > >
    > > Urgh.. that's plenty horrible. And I'm sure everybody plus kitchen sink
    > > has already asked why can't we just rcu free the thing unconditionally.
    > >
    > > Google only found me an earlier version of this same patch set, but I'm
    > > sure we've had that discussion many times over the past several years.
    > > The above and your follow up patch is just horrible.
    > >
    >
    > So on v3/v4 we got to doing that unconditionally for PREEMPT_RT, but per
    > [1] Wander went back to hand-fixing the problematic callsites.
    >
    > Now that I'm looking at it again, I couldn't find a concrete argument from
    > Oleg against doing this unconditionally - as Wander is pointing out in the
    > changelog and comments, reusing task_struct.rcu for that purpose is safe
    > (although not necessarily obviously so).
    >
    > Is this just miscommunication, or is there a genuine issue with doing this
    > unconditionally? As argued before, I'd also much rather have this be an
    > unconditional call_rcu() (regardless of context or PREEMPT_RT).
    >

    Yeah, I think it was a misunderstanding of mine.

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