Linux Ethernet Bridging - May 2015 - [Bridge] [RFC PATCH net-next] bridge: allow setting hash_max + multicast_router if interface is down

Network managers like netifd (used in OpenWRT for instance) try to
configure interface options after creation but before setting the
interface up.

Unfortunately the sysfs / bridge currently only allows to configure the
hash_max and multicast_router options when the bridge interface is up.
But since br_multicast_init() doesn't start any timers and only sets
default values and initializes timers it should be save to reconfigure
the default values after that, before things actually get active after
the bridge is set up.

With this patch hash_max and multicast_router attributes can be
changed even if the according bridge (port) is down, just like other
other bridge (port) attributes allow too.

Signed-off-by: Linus L?ssing <linus.luessing at c0d3.blue>
---

I'm currently a little unsure about a few things (that's why I'm
sending
this as an RFC):

* For i=br_multicast_init(), e=br_multicast_enable() and
  s=br_multicast_stop() is the order i->e->s->e->s->e->...
always
  ensured by the netdev API? Will this work even if I have
  many fast and wild calls to "ip link set up dev br0" and
  "ip link set down dev br0" and some changes to hash_max and
  multicast_router in between?

* Might calls to br_multicast_add_router() via br_multicast_enable_port()
  cause unintended side-effects?

* (maybe independent of this patch: )
  Can fast changes to the multicast_router attribute of a bridge (port)
  cause race conditions because a del_timer() instead of a
  del_timer_sync() is used in br_multicast_set_{port,}router()?
  (or: why does br_multicast_stop() use del_timer_sync() while
   br_multicast_disable_port() doesn't?)

* @Herbert: Do you remember whether there was a reason for
  checking netif_running() or the bridge port state which
  I might have overlooked?


PS: Running this patch didn't create any crashes for me so far
and multicast traffic still runs fine. No locust infestation in sight
yet either. Still careful though :P.

 net/bridge/br_multicast.c |   15 ++-------------
 1 file changed, 2 insertions(+), 13 deletions(-)

diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
index 2d69d5c..066199e 100644
--- a/net/bridge/br_multicast.c
+++ b/net/bridge/br_multicast.c
@@ -1775,8 +1775,6 @@ int br_multicast_set_router(struct net_bridge *br,
unsigned long val)
 	int err = -ENOENT;
 
 	spin_lock_bh(&br->multicast_lock);
-	if (!netif_running(br->dev))
-		goto unlock;
 
 	switch (val) {
 	case 0:
@@ -1793,7 +1791,6 @@ int br_multicast_set_router(struct net_bridge *br,
unsigned long val)
 		break;
 	}
 
-unlock:
 	spin_unlock_bh(&br->multicast_lock);
 
 	return err;
@@ -1802,18 +1799,15 @@ unlock:
 int br_multicast_set_port_router(struct net_bridge_port *p, unsigned long val)
 {
 	struct net_bridge *br = p->br;
-	int err = -ENOENT;
+	int err = 0;
 
 	spin_lock(&br->multicast_lock);
-	if (!netif_running(br->dev) || p->state == BR_STATE_DISABLED)
-		goto unlock;
 
 	switch (val) {
 	case 0:
 	case 1:
 	case 2:
 		p->multicast_router = val;
-		err = 0;
 
 		if (val < 2 && !hlist_unhashed(&p->rlist))
 			hlist_del_init_rcu(&p->rlist);
@@ -1834,7 +1828,6 @@ int br_multicast_set_port_router(struct net_bridge_port
*p, unsigned long val)
 		break;
 	}
 
-unlock:
 	spin_unlock(&br->multicast_lock);
 
 	return err;
@@ -1939,15 +1932,11 @@ unlock:
 
 int br_multicast_set_hash_max(struct net_bridge *br, unsigned long val)
 {
-	int err = -ENOENT;
+	int err = -EINVAL;
 	u32 old;
 	struct net_bridge_mdb_htable *mdb;
 
 	spin_lock_bh(&br->multicast_lock);
-	if (!netif_running(br->dev))
-		goto unlock;
-
-	err = -EINVAL;
 	if (!is_power_of_2(val))
 		goto unlock;
 
-- 
1.7.10.4

On Thu, May 21, 2015 at 04:59:22AM +0200, Linus L?ssing
wrote:
> 
> * For i=br_multicast_init(), e=br_multicast_enable() and
>   s=br_multicast_stop() is the order
i->e->s->e->s->e->... always
>   ensured by the netdev API? Will this work even if I have
br_multicast_init always happens first obviously.  But there is
no ordering requirement between e and s.
>   many fast and wild calls to "ip link set up dev br0" and
>   "ip link set down dev br0" and some changes to hash_max and
>   multicast_router in between?
The timer operations are all supposed to be idempotent.  So enabling
a port twice or stopping it twice should be OK.
> * Might calls to br_multicast_add_router() via br_multicast_enable_port()
>   cause unintended side-effects?
What do you mean? How does add_router get called from enable_port?
> * (maybe independent of this patch: )
>   Can fast changes to the multicast_router attribute of a bridge (port)
>   cause race conditions because a del_timer() instead of a
>   del_timer_sync() is used in br_multicast_set_{port,}router()?
>   (or: why does br_multicast_stop() use del_timer_sync() while
>    br_multicast_disable_port() doesn't?)
I don't think so.  Firstly everything should happen under the
multicast lock except for stop/del_port.  The latter rely on
those adding timers to check netif_running and port != disabled
to avoid races.

Apart from that you can always mod_timer twice or del_timer
twice so that's not really an issue.
> * @Herbert: Do you remember whether there was a reason for
>   checking netif_running() or the bridge port state which
>   I might have overlooked?
See above.  It's there so that you don't readd a timer when we're
calling del_timer_sync.  del_timer_sync has to be called without
the multicast lock so that's why we need another mechanism to
prevent the timers from being readded.

AFAICS the spots you patched aren't adding timers so they *should*
be OK.

Cheers,
-- 
Email: Herbert Xu <herbert at gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt