dlm: add rcu_barrier before destroy kmem cache

In the case we trigger dlm_free_rsb() that does a call_rcu() and the
responding kfree() of res_lvbptr and a kmem_cache_free() of the rsb
pointer we need to wait until this pending operation is done before
calling kmem_cache_destroy(). We doing that by using rcu_barrier() that
waits until all pending call_rcu() are done. This avoids that
kmem_cache_destroy() complains about active objects around that are not
being freed yet by call_rcu().

There is currently more discussions about to make this behaviour better,
see:

https://lore.kernel.org/netdev/20240609082726.32742-1-Julia.Lawall@inria.fr/

However this is only for call_rcu() if the callback calls
kmem_cache_destroy() only to replace it by kfree_rcu() call which has
currently some issue. This isn't our case because we also free the
res_lvbptr if being set.

For our case, to avoid the above race rcu_barrier() should be used before
calling kmem_cache_destroy() to be sure that there are no active objects
around. This is exactly what net/batman-adv is also doing before calling their
kmem_cache_destroy() in module unloading.

Fixes: 01fdeca1cc ("dlm: use rcu to avoid an extra rsb struct lookup")
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>

fs/dlm/memory.c

@@ -72,6 +72,8 @@ int __init dlm_memory_init(void)
 
 void dlm_memory_exit(void)
 {
+	rcu_barrier();
+
 	kmem_cache_destroy(writequeue_cache);
 	kmem_cache_destroy(mhandle_cache);
 	kmem_cache_destroy(msg_cache);