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

Paul E. McKenney
2017-05-11T14:50:49Z

On Thu, May 11, 2017 at 03:52:46PM +0200, Petr Mladek wrote:
> On Mon 2017-05-08 15:13:22, Steven Rostedt wrote:
> > On Mon, 8 May 2017 11:51:08 -0500
> > Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >
> > > Another idea would be to figure out a way to stop using RCU in
> > > klp_ftrace_handler() altogether.
> > >
> >
> > That may work if rcu_enter_irq() doesn't. But that's how NMIs use rcu.
>
> I am a bit confused by the above. Does it mean that RCU could not be
> used in NMI handlers?

Only RCU readers can be used in NMI handlers, that is, rcu_read_lock(),
rcu_read_unlock(), rcu_dereference(), and so on.

Thanx, Paul

> 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.
>
> 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();
>
>
>
> Best Regards,
> Petr
>