s390/pkey: Add comment about synchronize_rcu() to pkey base

Add a comment about the use of the synchronize_rcu() invocation.
There are two invocations of the synchronize_rcu() call in the
pkey base code. On one place it is optional but used to enforce
a fast path update to the other CPUs. As some people and code
checkers complain about this redundant invocation the suggestion
came up to add a comment to explain why the call is meaningful
at that place.

Closes: https://lore.kernel.org/linux-s390/20260313052312.2389-1-lirongqing@baidu.com/
Suggested-by: Li Rongqing <lirongqing@baidu.com>
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Author: hfreude

drivers/s390/crypto/pkey_base.c

@@ -60,6 +60,13 @@ int pkey_handler_register(struct pkey_handler *handler)
 
 	list_add_rcu(&handler->list, &handler_list);
 	spin_unlock(&handler_list_write_lock);
+	/*
+	 * Fast path to push the info about the updated list to the other
+	 * CPUs. If removed, the other CPUs may get the updated list when the
+	 * RCU context is synched. As this code is in general not performance
+	 * critical and the list update mostly only occurs at the early time in
+	 * system startup the focus is on concurrency versus performance.
+	 */
 	synchronize_rcu();
 
 	module_put(handler->module);