Messages in this thread | | | Date | Thu, 11 Feb 2016 17:10:00 +0100 | From | Petr Mladek <> | Subject | Re: [RFC][PATCH v3 4/4] printk: set may_schedule for some of console_trylock callers |
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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
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