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

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

[Lin Liu]

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

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

[Jakub Kicinski]

On Tue, 29 Nov 2022 06:17:02 +0000 Lin Liu wrote:
> 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

You can trim all the ' ? ' entries from the stack trace, 
and the time stamps, FWIW. Makes it easier to read.

> [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
> 

Since this is a fix please add a Fixes tag, and add [PATCH net]
to the subject.

> 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();

Now all callers of xennet_disconnect_backend() destroy queues soon
after, can we just move the destroy queues into disconnect ?

>  	return 0;
>  }
>