Re: [PATCH 2/3] livepatch/rcu: Warn when system consistency is broken in RCU code

Josh Poimboeuf
2017-05-11T15:28:01Z

On Thu, May 11, 2017 at 03:52:46PM +0200, Petr Mladek wrote:
> Anyway, a crazy idea is to use the livepatch consistency model instead
> of RCU to protect the function stack. The model makes sure that all
> tasks, including the idle ones, were not running any patched function
> (and their ftrace handlers) at some point. It should be safe
> but I am not sure if it is worth it.

http://i3.kym-cdn.com/photos/images/original/000/173/580/Wat.jpg

> Alternatively, it might be enough to use the probably more lightwight
> solution that is used when ftrace handlers are deregistered, I mean:
>
> /*
> * We need to do a hard force of sched synchronization.
> * This is because we use preempt_disable() to do RCU, but
> * the function tracers can be called where RCU is not watching
> * (like before user_exit()). We can not rely on the RCU
> * infrastructure to do the synchronization, thus we must do it
> * ourselves.
> */
> schedule_on_each_cpu(ftrace_sync);
>
> /*
> * When the kernel is preeptive, tasks can be preempted
> * while on a ftrace trampoline. Just scheduling a task on
> * a CPU is not good enough to flush them. Calling
> * synchornize_rcu_tasks() will wait for those tasks to
> * execute and either schedule voluntarily or enter user space.
> */
> if (IS_ENABLED(CONFIG_PREEMPT))
> synchronize_rcu_tasks();

I couldn't grok the first idea, but this one sounds promising...

--
Josh