On Saturday 01 December 2012 18:29:51 Simon Wunderlich wrote:

If rtnl_trylock() is used to prevent circular dependencies, rtnl_unlock()
may destroy this attempt because it first unlocks rtnl_mutex but may
lock it again later. The callgraph roughly looks like:

rtnl_unlock()
   netdev_run_todo()
      __rtnl_unlock()
      netdev_wait_allrefs()
         rtnl_lock()
         ...
         __rtnl_unlock()

There are two users which have possible deadlocks and are fixed in this
patch: batman-adv and bridge. Their problematic access pattern is the same:

notifier_call handler:
 * holds rtnl lock when called
 * calls sysfs code at some point (acquiring sysfs locks)

sysfs code:
 * holds sysfs lock when called
 * calls rtnl_trylock() and rtnl_unlock(), rtnl_unlock() calls rtnl_lock
   implicitly

Fix this by exporting __rtnl_unlock() to just unlock the mutex without
implicitly locking again.

Reported-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <redacted>

---
Of course, an alternative would be to not lock again after unlocking
within rtnl_unlock(), or put the todo handling into the locked section.
I'm not familiar enough with this code to know what would be best.

There are others using rtnl_trylock(), but I'm not sure if they
are affected.

At least they look like they have a problem in a parallel user scenario 
involving another lock and locking order (most of them s_active or a device 
lock). So just to list the places and poke some other users. They can better 
decide for themself because they know the code.

drivers/infiniband/ulp/ipoib/ipoib_cm.c:  if (!rtnl_trylock())
drivers/infiniband/ulp/ipoib/ipoib_vlan.c:        if (!rtnl_trylock())
drivers/infiniband/ulp/ipoib/ipoib_vlan.c:        if (!rtnl_trylock())
drivers/net/bonding/bond_alb.c:                   if (!rtnl_trylock()) {
drivers/net/bonding/bond_main.c:          if (!rtnl_trylock()) {
drivers/net/bonding/bond_main.c:          if (!rtnl_trylock()) {
drivers/net/bonding/bond_main.c:          if (!rtnl_trylock()) {
drivers/net/bonding/bond_main.c:          if (!rtnl_trylock()) {
drivers/net/bonding/bond_sysfs.c: if (!rtnl_trylock())
drivers/net/bonding/bond_sysfs.c: if (!rtnl_trylock())
drivers/net/bonding/bond_sysfs.c: if (!rtnl_trylock())
drivers/net/bonding/bond_sysfs.c: if (!rtnl_trylock())
drivers/net/bonding/bond_sysfs.c: if (!rtnl_trylock())
drivers/net/bonding/bond_sysfs.c: if (!rtnl_trylock())
drivers/net/ethernet/chelsio/cxgb3/cxgb3_main.c:  if (!rtnl_trylock())    /* 
synchronize with ifdown */
drivers/s390/net/qeth_l2_main.c:          if (rtnl_trylock()) {
drivers/s390/net/qeth_l3_main.c:          if (rtnl_trylock()) {
net/bridge/br_sysfs_br.c: if (!rtnl_trylock())
net/bridge/br_sysfs_if.c:         if (!rtnl_trylock())
net/core/net-sysfs.c:     if (!rtnl_trylock())
net/core/net-sysfs.c:     if (!rtnl_trylock())
net/core/net-sysfs.c:     if (!rtnl_trylock())
net/core/net-sysfs.c:     if (!rtnl_trylock())
net/core/net-sysfs.c:     if (!rtnl_trylock())
net/ipv4/devinet.c:                       if (!rtnl_trylock()) {
net/ipv6/addrconf.c:      if (!rtnl_trylock())
net/ipv6/addrconf.c:      if (!rtnl_trylock())

Kind regards,
	Sven