[RFC PATCH 4/6] ppp: invert lock ordering between ppp_mutex and rtnl_lock

[Guillaume Nault]

Since ppp_create_interface() is the only place[1] holding both ppp_mutex
and rtnl_lock(), we can reverse lock ordering to allow taking ppp_mutex
when rtnl_lock is already held.
This will be necessary for creating PPP interfaces from an rtnetlink
callback.

[1]: There may actually be exceptions. When operating on PPP channels,
ppp_ioctl() can call lower level ioctl callbacks (chan->ops->ioctl())
with ppp_mutex locked. More review would be necessary to ensure no such
callback would ever call rtnl_lock(). OTOH, with proper locking inside
the ->ioctl() callbacks, ppp_mutex might be removed entirely from this
part of the code.

Signed-off-by: Guillaume Nault <g.na...@alphalink.fr>
---
 drivers/net/ppp/ppp_generic.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index c81e257..8aaedb8 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -2796,8 +2796,8 @@ static int ppp_create_interface(struct net *net, struct 
file *file, int *unit)
         */
        dev_net_set(dev, net);
 
-       mutex_lock(&ppp_mutex);
        rtnl_lock();
+       mutex_lock(&ppp_mutex);
        mutex_lock(&pn->all_ppp_mutex);
 
        if (file->private_data) {
@@ -2847,15 +2847,15 @@ static int ppp_create_interface(struct net *net, struct 
file *file, int *unit)
        atomic_inc(&ppp_unit_count);
 
        mutex_unlock(&pn->all_ppp_mutex);
-       rtnl_unlock();
        mutex_unlock(&ppp_mutex);
+       rtnl_unlock();
 
        return 0;
 
 out2:
        mutex_unlock(&pn->all_ppp_mutex);
-       rtnl_unlock();
        mutex_unlock(&ppp_mutex);
+       rtnl_unlock();
        free_netdev(dev);
 out1:
        return ret;
-- 
2.8.0.rc3