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

[Wander Lairson Costa]

On Thu, May 04, 2023 at 10:42:29AM +0200, 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.
> 
> It requires users to know the RT specific context and gives them no help
> what so ever for !RT builds.
> 
> 

No problem, I will send a new version doing it unconditionally.