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Date:   Tue, 12 Oct 2021 08:29:20 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Miroslav Benes <mbenes@suse.cz>
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        live-patching@vger.kernel.org
Subject: Re: [PATCH 1/2] ftrace: disable preemption on the testing of
 recursion
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On Tue, 12 Oct 2021 14:24:43 +0200 (CEST)
Miroslav Benes <mbenes@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