Messages in this thread | | | From | Wander Lairson Costa <> | Date | Mon, 6 Feb 2023 15:36:38 -0300 | Subject | Re: [PATCH v4] kernel/fork: beware of __put_task_struct calling context |
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On Mon, Feb 6, 2023 at 1:04 PM, Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote: > > On 2023-02-06 16:27:12 [+0100], Oleg Nesterov wrote: > > On 02/06, Sebastian Andrzej Siewior wrote: > > > > > > On 2023-02-06 10:04:47 [-0300], Wander Lairson Costa wrote: > > > > > > > @@ -857,6 +857,29 @@ void __put_task_struct(struct task_struct *tsk) > > > … > > > > +void __put_task_struct(struct task_struct *tsk) > > > > +{ > > > > + if (IS_ENABLED(CONFIG_PREEMPT_RT) && (!preemptible() || !in_task())) > > > > > > Is it safe to use the rcu member in any case? > > > > I thinks it is safe but deserves a comment. I guess Wander misunderstood > > me when I asked him to do this... > > > > __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. > > Sounds good. > > > > If so why not use it > > > unconditionally? > > > > performance ? > > All the free() part is moved from the caller into rcu. > > > > > And... I still don't like the name of delayed_put_task_struct_rcu() to me > > ___put_task_struct_rcu() looks a bit less confusing, note that we already > > have delayed_put_task_struct(). But this is minor. > > So if we do it unconditionally then we could get rid of > put_task_struct_rcu_user(). > Otherwise we could use put_task_struct_rcu_user() in that timer > callback because it will lead to lockdep warnings once printk is fixed.
put_task_struct_rcu_user() calls delayed_put_task_struct(), which does more than just call __put_task_struct(). I tried this approach at the beginning, but I got another splat (unfortunately, I don't remember where).
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