On Fri 2016-02-12 00:02:17, Sergey Senozhatsky wrote:
> Hello Petr,
>
> On (02/11/16 15:41), Petr Mladek wrote:
> [..]
> > > + console_may_schedule = !oops_in_progress &&
> > > + preemptible() &&
> > > + !rcu_preempt_depth();
> > > return 1;
> >
> > We discussed this a lot but I am still a bit nervous ;-)
>
> sure, no prob :-)
>
> > Avoid scheduling when oops_in_progress makes sense.
> >
> > preemptible() takes care of preemption and IRQ contexts.
> > The comment above explains that it is safe to use here.
> >
> > The check for rcu_preempt_depth() makes sense. But is it
> > safe, please?
> >
> > rcu_preempt_depth() returns 0 if CONFIG_PREEMPT_RCU is not
> > enabled. It means that you are not able to detect RCU read
> > section and it might cause problems.
>
> well, I believe it's ok. __rcu_read_lock() for CONFIG_PREEMPT_RCU
> does current->rcu_read_lock_nesting++, so rcu_preempt_depth() works
> as expected. otherwise, for !CONFIG_PREEMPT_RCU kernel,
> __rcu_read_lock() does
>
> if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
> preempt_disable()
>
>
> - if we run "CONFIG_PREEMPT_RCU" then rcu_preempt_depth()
> works here.
>
> - if we run "!CONFIG_PREEMPT_RCU && CONFIG_PREEMPT_COUNT"
> then preemptible() works for us
>
> - if we run "!CONFIG_PREEMPT_RCU && !CONFIG_PREEMPT_COUNT"
> then preemptible() is always 0.
I feel convinced. But we should somehow document it. I think how
to do it effectively. I think that the following text would help
me if I read it:
/*
* Safe context for rescheduling is detected only when
* PREEMPT_COUNT is enabled. preemptible() always returns
* false otherwise.
*
* RCU read sections must be detected separately. They
* have a separate preemption counter when PREEMPT_RCU
* is enabled.
*/
I wanted to highlight why exactly the check returns 0 in !PREEMPT_COUNT
kernel. I missed this a bit in you original comment. But feel free
to change it as you like.
Best Regards,
Petr
