Messages in this thread | | | Date | Thu, 4 May 2023 09:24:21 -0300 | From | Wander Lairson Costa <> | Subject | Re: [PATCH v7 2/3] sched/task: Add the put_task_struct_atomic_safe() function |
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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.
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