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

From: Jakub Kicinski <[email-masked]>
Date: Wed, 30 Nov 2022 21:26:45 -0800

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;
>  }