ppp_dev_uninit() locks all_ppp_mutex while under rtnl mutex protection.
ppp_create_interface() must then lock these mutexes in that same order
to avoid possible deadlock.

[  120.880011] ======================================================
[  120.880011] [ INFO: possible circular locking dependency detected ]
[  120.880011] 4.2.0 #1 Not tainted
[  120.880011] -------------------------------------------------------
[  120.880011] ppp-apitest/15827 is trying to acquire lock:
[  120.880011]  (&pn->all_ppp_mutex){+.+.+.}, at: [<ffffffffa0145f56>] ppp_dev_uninit+0x64/0xb0 [ppp_generic]
[  120.880011]
[  120.880011] but task is already holding lock:
[  120.880011]  (rtnl_mutex){+.+.+.}, at: [<ffffffff812e4255>] rtnl_lock+0x12/0x14
[  120.880011]
[  120.880011] which lock already depends on the new lock.
[  120.880011]
[  120.880011]
[  120.880011] the existing dependency chain (in reverse order) is:
[  120.880011]
[  120.880011] -> #1 (rtnl_mutex){+.+.+.}:
[  120.880011]        [<ffffffff81073a6f>] lock_acquire+0xcf/0x10e
[  120.880011]        [<ffffffff813ab18a>] mutex_lock_nested+0x56/0x341
[  120.880011]        [<ffffffff812e4255>] rtnl_lock+0x12/0x14
[  120.880011]        [<ffffffff812d9d94>] register_netdev+0x11/0x27
[  120.880011]        [<ffffffffa0147b17>] ppp_ioctl+0x289/0xc98 [ppp_generic]
[  120.880011]        [<ffffffff8113b367>] do_vfs_ioctl+0x4ea/0x532
[  120.880011]        [<ffffffff8113b3fd>] SyS_ioctl+0x4e/0x7d
[  120.880011]        [<ffffffff813ad7d7>] entry_SYSCALL_64_fastpath+0x12/0x6f
[  120.880011]
[  120.880011] -> #0 (&pn->all_ppp_mutex){+.+.+.}:
[  120.880011]        [<ffffffff8107334e>] __lock_acquire+0xb07/0xe76
[  120.880011]        [<ffffffff81073a6f>] lock_acquire+0xcf/0x10e
[  120.880011]        [<ffffffff813ab18a>] mutex_lock_nested+0x56/0x341
[  120.880011]        [<ffffffffa0145f56>] ppp_dev_uninit+0x64/0xb0 [ppp_generic]
[  120.880011]        [<ffffffff812d5263>] rollback_registered_many+0x19e/0x252
[  120.880011]        [<ffffffff812d5381>] rollback_registered+0x29/0x38
[  120.880011]        [<ffffffff812d53fa>] unregister_netdevice_queue+0x6a/0x77
[  120.880011]        [<ffffffffa0146a94>] ppp_release+0x42/0x79 [ppp_generic]
[  120.880011]        [<ffffffff8112d9f6>] __fput+0xec/0x192
[  120.880011]        [<ffffffff8112dacc>] ____fput+0x9/0xb
[  120.880011]        [<ffffffff8105447a>] task_work_run+0x66/0x80
[  120.880011]        [<ffffffff81001801>] prepare_exit_to_usermode+0x8c/0xa7
[  120.880011]        [<ffffffff81001900>] syscall_return_slowpath+0xe4/0x104
[  120.880011]        [<ffffffff813ad931>] int_ret_from_sys_call+0x25/0x9f
[  120.880011]
[  120.880011] other info that might help us debug this:
[  120.880011]
[  120.880011]  Possible unsafe locking scenario:
[  120.880011]
[  120.880011]        CPU0                    CPU1
[  120.880011]        ----                    ----
[  120.880011]   lock(rtnl_mutex);
[  120.880011]                                lock(&pn->all_ppp_mutex);
[  120.880011]                                lock(rtnl_mutex);
[  120.880011]   lock(&pn->all_ppp_mutex);
[  120.880011]
[  120.880011]  *** DEADLOCK ***

Fixes: 8cb775bc ("ppp: fix device unregistration upon netns deletion")
Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>