Re: [PATCH 04/13] module: use RCU to synchronize find_module

[Jessica Yu]

+++ Miroslav Benes [29/01/21 16:29 +0100]:
> On Thu, 28 Jan 2021, Christoph Hellwig wrote:
>
> > 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.
>
> > @@ -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()