Merge branch 'rtnetlink-rework-handler-registration'

Florian Westphal says:

====================
rtnetlink: rework handler (un)registering

Peter Zijlstra reported (referring to commit 019a316,
"rtnetlink: add reference counting to prevent module unload while dump is in progress"):

 1) it not in fact a refcount, so using refcount_t is silly
 2) there is a distinct lack of memory barriers, so we can easily
    observe the decrement while the msg_handler is still in progress.
 3) waiting with a schedule()/yield() loop is complete crap and subject
    life-locks, imagine doing that rtnl_unregister_all() from a RT task.

In ancient times rtnetlink exposed a statically-sized table with
preset doit/dumpit handlers to be called for a protocol/type pair.

Later the rtnl_register interface was added and the table was allocated
on demand.  Eventually these were also used by modules.

Problem is that nothing prevents module unload while a netlink dump
is in progress.  netlink dumps can be span multiple recv calls and
netlink core saves the to-be-repeated dumper address for later invocation.

To prevent rmmod the netlink core expects callers to pass in the owning
module so a reference can be taken.

So far rtnetlink wasn't doing this, add new interface to pass THIS_MODULE.
Moreover, when converting parts of the rtnetlink handling to rcu this code
gained way too many READ_ONCE spots, remove them and the extra refcounting.

Take a module reference when running dumpit and doit callbacks
and never alter content of rtnl_link structures after they have been
published via rcu_assign_pointer.

Based partially on earlier patch from Peter.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/rtnetlink.h

@@ -13,10 +13,10 @@ enum rtnl_link_flags {
 	RTNL_FLAG_DOIT_UNLOCKED = 1,
 };
 
-int __rtnl_register(int protocol, int msgtype,
-		    rtnl_doit_func, rtnl_dumpit_func, unsigned int flags);
 void rtnl_register(int protocol, int msgtype,
 		   rtnl_doit_func, rtnl_dumpit_func, unsigned int flags);
+int rtnl_register_module(struct module *owner, int protocol, int msgtype,
+			 rtnl_doit_func, rtnl_dumpit_func, unsigned int flags);
 int rtnl_unregister(int protocol, int msgtype);
 void rtnl_unregister_all(int protocol);
 

net/bridge/br_mdb.c

@@ -760,9 +760,9 @@ static int br_mdb_del(struct sk_buff *skb, struct nlmsghdr *nlh,
 
 void br_mdb_init(void)
 {
-	rtnl_register(PF_BRIDGE, RTM_GETMDB, NULL, br_mdb_dump, 0);
-	rtnl_register(PF_BRIDGE, RTM_NEWMDB, br_mdb_add, NULL, 0);
-	rtnl_register(PF_BRIDGE, RTM_DELMDB, br_mdb_del, NULL, 0);
+	rtnl_register_module(THIS_MODULE, PF_BRIDGE, RTM_GETMDB, NULL, br_mdb_dump, 0);
+	rtnl_register_module(THIS_MODULE, PF_BRIDGE, RTM_NEWMDB, br_mdb_add, NULL, 0);
+	rtnl_register_module(THIS_MODULE, PF_BRIDGE, RTM_DELMDB, br_mdb_del, NULL, 0);
 }
 
 void br_mdb_uninit(void)

net/can/gw.c

@@ -1014,6 +1014,8 @@ static struct pernet_operations cangw_pernet_ops = {
 
 static __init int cgw_module_init(void)
 {
+	int ret;
+
 	/* sanitize given module parameter */
 	max_hops = clamp_t(unsigned int, max_hops, CGW_MIN_HOPS, CGW_MAX_HOPS);
 
@@ -1031,15 +1033,19 @@ static __init int cgw_module_init(void)
 	notifier.notifier_call = cgw_notifier;
 	register_netdevice_notifier(&notifier);
 
-	if (__rtnl_register(PF_CAN, RTM_GETROUTE, NULL, cgw_dump_jobs, 0)) {
+	ret = rtnl_register_module(THIS_MODULE, PF_CAN, RTM_GETROUTE,
+				   NULL, cgw_dump_jobs, 0);
+	if (ret) {
 		unregister_netdevice_notifier(&notifier);
 		kmem_cache_destroy(cgw_cache);
 		return -ENOBUFS;
 	}
 
-	/* Only the first call to __rtnl_register can fail */
-	__rtnl_register(PF_CAN, RTM_NEWROUTE, cgw_create_job, NULL, 0);
-	__rtnl_register(PF_CAN, RTM_DELROUTE, cgw_remove_job, NULL, 0);
+	/* Only the first call to rtnl_register_module can fail */
+	rtnl_register_module(THIS_MODULE, PF_CAN, RTM_NEWROUTE,
+			     cgw_create_job, NULL, 0);
+	rtnl_register_module(THIS_MODULE, PF_CAN, RTM_DELROUTE,
+			     cgw_remove_job, NULL, 0);
 
 	return 0;
 }