[PATCH] drivers/net/netfront: Fix NULL sring after live migration

[Lin Liu <lin.liu@citrix.com>]

A NAPI is setup for each network sring to poll data to kernel
The sring with source host is destroyed before live migration and
new sring with target host is setup after live migration.
The NAPI for the old sring is not deleted until setup new sring
with target host after migration. With busy_poll/busy_read enabled,
the NAPI can be polled before got deleted when resume VM.

[50116.602938] BUG: unable to handle kernel NULL pointer dereference at
0000000000000008
[50116.603047] IP: xennet_poll+0xae/0xd20
[50116.603090] PGD 0 P4D 0
[50116.603118] Oops: 0000 [#1] SMP PTI
[50116.604624] Call Trace:
[50116.604674]  ? finish_task_switch+0x71/0x230
[50116.604745]  ? timerqueue_del+0x1d/0x40
[50116.604807]  ? hrtimer_try_to_cancel+0xb5/0x110
[50116.604882]  ? xennet_alloc_rx_buffers+0x2a0/0x2a0
[50116.604958]  napi_busy_loop+0xdb/0x270
[50116.605017]  sock_poll+0x87/0x90
[50116.605066]  do_sys_poll+0x26f/0x580
[50116.605125]  ? tracing_map_insert+0x1d4/0x2f0
[50116.605196]  ? event_hist_trigger+0x14a/0x260
...
[50116.613598]  ? finish_task_switch+0x71/0x230
[50116.614131]  ? __schedule+0x256/0x890
[50116.614640]  ? recalc_sigpending+0x1b/0x50
[50116.615144]  ? xen_sched_clock+0x15/0x20
[50116.615643]  ? __rb_reserve_next+0x12d/0x140
[50116.616138]  ? ring_buffer_lock_reserve+0x123/0x3d0
[50116.616634]  ? event_triggers_call+0x87/0xb0
[50116.617138]  ? trace_event_buffer_commit+0x1c4/0x210
[50116.617625]  ? xen_clocksource_get_cycles+0x15/0x20
[50116.618112]  ? ktime_get_ts64+0x51/0xf0
[50116.618578]  SyS_ppoll+0x160/0x1a0
[50116.619029]  ? SyS_ppoll+0x160/0x1a0
[50116.619475]  do_syscall_64+0x73/0x130
[50116.619901]  entry_SYSCALL_64_after_hwframe+0x41/0xa6
...
[50116.806230] RIP: xennet_poll+0xae/0xd20 RSP: ffffb4f041933900
[50116.806772] CR2: 0000000000000008
[50116.807337] ---[ end trace f8601785b354351c ]---

xen frontend should remove the NAPIs for the old srings before live
migration as the bond srings are destroyed

There is a tiny window between the srings are set to NULL and
the NAPIs are disabled, It is safe as the NAPI threads are still
frozen at that time

Signed-off-by: Lin Liu <lin.liu@citrix.com>
---
 drivers/net/xen-netfront.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 9af2b027c19c..dc404e05970c 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -1862,6 +1862,12 @@ static int netfront_resume(struct xenbus_device *dev)
 	netif_tx_unlock_bh(info->netdev);
 
 	xennet_disconnect_backend(info);
+
+	rtnl_lock();
+	if (info->queues)
+		xennet_destroy_queues(info);
+	rtnl_unlock();
+
 	return 0;
 }
 
-- 
2.17.1