Re: [PATCH 1/2] ftrace: disable preemption on the testing of recursion

On Tue, 12 Oct 2021 14:24:43 +0200 (CEST)
Miroslav Benes [off-list ref] wrote:

quoted

+++ 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