Messages in this thread | | | Date | Wed, 15 Apr 2020 15:54:50 -0700 | From | "Paul E. McKenney" <> | Subject | Re: On trace_*_rcuidle functions in modules |
| |
On Wed, Apr 15, 2020 at 06:51:21PM -0400, Steven Rostedt wrote: > On Wed, 15 Apr 2020 15:04:59 -0700 > "Paul E. McKenney" <paulmck@kernel.org> wrote: > > > On Wed, Apr 15, 2020 at 05:49:18PM -0400, Steven Rostedt wrote: > > > On Wed, 15 Apr 2020 14:02:04 -0700 > > > John Stultz <john.stultz@linaro.org> wrote: > > > > > > > > > > > So in my case your concerns may not be a problem, but I guess > > > > generally it might. Though I'd hope the callback would be unregistered > > > > (and whatever waiting for the grace period to complete be done) before > > > > the module removal is complete. But maybe I'm still missing your > > > > point? > > > > > > Hmm, you may have just brought up a problem here... > > > > > > You're saying that cpu_pm_register_notifier() callers are called from non > > > RCU watching context? If that's the case, we have this: > > > > > > int cpu_pm_unregister_notifier(struct notifier_block *nb) > > > { > > > return atomic_notifier_chain_unregister(&cpu_pm_notifier_chain, nb); > > > } > > > > > > And this: > > > > > > int atomic_notifier_chain_unregister(struct atomic_notifier_head *nh, > > > struct notifier_block *n) > > > { > > > unsigned long flags; > > > int ret; > > > > > > spin_lock_irqsave(&nh->lock, flags); > > > ret = notifier_chain_unregister(&nh->head, n); > > > spin_unlock_irqrestore(&nh->lock, flags); > > > synchronize_rcu(); > > > return ret; > > > } > > > > > > Which means that if something registered a cpu_pm notifier, then > > > unregistered it, and freed whatever the notifier accesses, then there's a > > > chance that the synchronize_rcu() can return before the called notifier > > > finishes, and anything that notifier accesses could have been freed. > > > > > > I believe that module code should not be able to be run in RCU non watching > > > context, and neither should notifiers. I think we just stumbled on a bug. > > > > > > Paul? > > > > Or we say that such modules cannot be unloaded. Or that such modules' > > exit handlers, after disentangling themselves from the idle loop, must > > invoke synchronize_rcu_rude() or similar, just as modules that use > > call_rcu() are currently required to invoke rcu_barrier(). > > > > Or is it possible to upgrade the protection that modules use? > > > > My guess is that invoking rcu_irq_enter() and rcu_irq_exit() around every > > potential call into module code out of the PM code is a non-starter, > > but I cannot prove that either way. > > > > No this has nothing to do with modules. This is a bug right now with the > cpu_pm notifier (after looking at the code, it's not a bug right now, see > below). > > Say you have something that allocates some data and registers a > callback to the cpu_pm notifier that access that data. Then for some > reason, you want to remove that notifier and free the data. Usually you > would do: > > cpu_pm_unregister_notifier(my_notifier); > kfree(my_data); > > But the problem is that the callback of that my_notifier could be executing > in a RCU non-watching space, and the cpu_pm_unregister_notifier() can > return before the my_notifier is done, and the my_data is freed. Then the > callback for the my_notifier could still be accessing the my_data. > > > /me goes and reads the code and sees this is not an issue, and you can > ignore the above concern. > > I was about to suggest a patch, but that has already been written... > > 313c8c16ee62b ("PM / CPU: replace raw_notifier with atomic_notifier") > > Which surrounds the notifier callbacks with rcu_irq_enter_irqson() > > Which means that if John moves the code to use the notifier, then he could > also remove the _rcuidle(), because RCU will be watching.
Whew!!! ;-)
Thanx, Paul
| |