On Tue, 12 Oct 2021 14:24:43 +0200 (CEST)
Miroslav Benes <mbe...@suse.cz> wrote:
> > +++ b/kernel/livepatch/patch.c
> > @@ -52,11 +52,6 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> > bit = ftrace_test_recursion_trylock(ip, parent_ip);
> > if (WARN_ON_ONCE(bit < 0))
> > return;
> > - /*
> > - * A variant of synchronize_rcu() is used to allow patching functions
> > - * where RCU is not watching, see klp_synchronize_transition().
> > - */
> > - preempt_disable_notrace();
> >
> > func = list_first_or_null_rcu(&ops->func_stack, struct klp_func,
> > stack_node);
> > @@ -120,7 +115,6 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> > klp_arch_set_pc(fregs, (unsigned long)func->new_func);
> >
> > unlock:
> > - preempt_enable_notrace();
> > ftrace_test_recursion_unlock(bit);
> > }
>
> I don't like this change much. We have preempt_disable there not because
> of ftrace_test_recursion, but because of RCU. ftrace_test_recursion was
> added later. Yes, it would work with the change, but it would also hide
> things which should not be hidden in my opinion.
Agreed, but I believe the change is fine, but requires a nice comment to
explain what you said above.
Thus, before the "ftrace_test_recursion_trylock()" we need:
/*
* The ftrace_test_recursion_trylock() will disable preemption,
* which is required for the variant of synchronize_rcu() that is
* used to allow patching functions where RCU is not watching.
* See klp_synchronize_transition() for more details.
*/
-- Steve
