macvtap: fix two races

Since commit ac4e4af1 ("macvtap: Consistently use rcu functions"),
Thomas gets two different warnings :

BUG: using smp_processor_id() in preemptible [00000000] code: vhost-45891/45892
caller is macvtap_do_read+0x45c/0x600 [macvtap]
CPU: 1 PID: 45892 Comm: vhost-45891 Not tainted 3.11.0-bisecttest #13
Call Trace:
([<00000000001126ee>] show_trace+0x126/0x144)
 [<00000000001127d2>] show_stack+0xc6/0xd4
 [<000000000068bcec>] dump_stack+0x74/0xd8
 [<0000000000481066>] debug_smp_processor_id+0xf6/0x114
 [<000003ff802e9a18>] macvtap_do_read+0x45c/0x600 [macvtap]
 [<000003ff802e9c1c>] macvtap_recvmsg+0x60/0x88 [macvtap]
 [<000003ff80318c5e>] handle_rx+0x5b2/0x800 [vhost_net]
 [<000003ff8028f77c>] vhost_worker+0x15c/0x1c4 [vhost]
 [<000000000015f3ac>] kthread+0xd8/0xe4
 [<00000000006934a6>] kernel_thread_starter+0x6/0xc
 [<00000000006934a0>] kernel_thread_starter+0x0/0xc

And

BUG: using smp_processor_id() in preemptible [00000000] code: vhost-45897/45898
caller is macvlan_start_xmit+0x10a/0x1b4 [macvlan]
CPU: 1 PID: 45898 Comm: vhost-45897 Not tainted 3.11.0-bisecttest #16
Call Trace:
([<00000000001126ee>] show_trace+0x126/0x144)
 [<00000000001127d2>] show_stack+0xc6/0xd4
 [<000000000068bdb8>] dump_stack+0x74/0xd4
 [<0000000000481132>] debug_smp_processor_id+0xf6/0x114
 [<000003ff802b72ca>] macvlan_start_xmit+0x10a/0x1b4 [macvlan]
 [<000003ff802ea69a>] macvtap_get_user+0x982/0xbc4 [macvtap]
 [<000003ff802ea92a>] macvtap_sendmsg+0x4e/0x60 [macvtap]
 [<000003ff8031947c>] handle_tx+0x494/0x5ec [vhost_net]
 [<000003ff8028f77c>] vhost_worker+0x15c/0x1c4 [vhost]
 [<000000000015f3ac>] kthread+0xd8/0xe4
 [<000000000069356e>] kernel_thread_starter+0x6/0xc
 [<0000000000693568>] kernel_thread_starter+0x0/0xc
2 locks held by vhost-45897/45898:
 #0:  (&vq->mutex){+.+.+.}, at: [<000003ff8031903c>] handle_tx+0x54/0x5ec [vhost_net]
 #1:  (rcu_read_lock){.+.+..}, at: [<000003ff802ea53c>] macvtap_get_user+0x824/0xbc4 [macvtap]

In the first case, macvtap_put_user() calls macvlan_count_rx()
in a preempt-able context, and this is not allowed.

In the second case, macvtap_get_user() calls
macvlan_start_xmit() with BH enabled, and this is not allowed.

Reported-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Bisected-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Tested-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Cc: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>