On Mon, 1 Feb 2021, Jessica Yu wrote:

+++ 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.

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@@ -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()

Ok, thanks to both for checking and explanation.

Ack to the patch then.

Miroslav