+++ Miroslav Benes [29/01/21 16:29 +0100]:

On Thu, 28 Jan 2021, Christoph Hellwig wrote:

quoted

Allow for a RCU-sched critical section around find_module, following
the lower level find_module_all helper, and switch the two callers
outside of module.c to use such a RCU-sched critical section instead
of module_mutex.

That's a nice idea.

quoted

@@ -57,7 +58,7 @@ static void klp_find_object_module(struct klp_object *obj)
 	if (!klp_is_module(obj))
 		return;

-	mutex_lock(&module_mutex);
+	rcu_read_lock_sched();
 	/*
 	 * We do not want to block removal of patched modules and therefore
 	 * we do not take a reference here. The patches are removed by

@@ -74,7 +75,7 @@ static void klp_find_object_module(struct klp_object *obj)
 	if (mod && mod->klp_alive)

RCU always baffles me a bit, so I'll ask. Don't we need
rcu_dereference_sched() here? "mod" comes from a RCU-protected list, so I
wonder.

Same here :-) I had to double check the RCU documentation. For our
modules list case I believe the rcu list API should take care of that
for us. Worth noting is this snippet from Documentation/RCU/whatisRCU.txt:

    rcu_dereference() is typically used indirectly, via the _rcu
    list-manipulation primitives, such as list_for_each_entry_rcu()