Re: [PATCH v7 2/3] sched/task: Add the put_task_struct_atomic_safe() function

[Wander Lairson Costa]

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.