Subject: Re: [PATCH net-next 1/1] netvsc: fix rtnl deadlock on unregister of vf

From: Vitaly Kuznetsov <[email-masked]>
Date: Mon, 07 Aug 2017 15:37:49 +0200

Vitaly Kuznetsov <[email-masked]> writes:

> Stephen Hemminger <[email-masked]> writes:
>
>> With new transparent VF support, it is possible to get a deadlock
>> when some of the deferred work is running and the unregister_vf
>> is trying to cancel the work element. The solution is to use
>> trylock and reschedule (similar to bonding and team device).
>>
>> Reported-by: Vitaly Kuznetsov <[email-masked]>
>> Fixes: 0c195567a8f6 ("netvsc: transparent VF management")
>> Signed-off-by: Stephen Hemminger <[email-masked]>
>> ---
>>  drivers/net/hyperv/netvsc_drv.c | 12 ++++++++++--
>>  1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/net/hyperv/netvsc_drv.c b/drivers/net/hyperv/netvsc_drv.c
>> index c71728d82049..e75c0f852a63 100644
>> --- a/drivers/net/hyperv/netvsc_drv.c
>> +++ b/drivers/net/hyperv/netvsc_drv.c
>> @@ -1601,7 +1601,11 @@ static void netvsc_vf_setup(struct work_struct *w)
>>  	struct net_device *ndev = hv_get_drvdata(ndev_ctx->device_ctx);
>>  	struct net_device *vf_netdev;
>>
>> -	rtnl_lock();
>> +	if (!rtnl_trylock()) {
>> +		schedule_work(w);
>> +		return;
>> +	}
>> +
>>  	vf_netdev = rtnl_dereference(ndev_ctx->vf_netdev);
>>  	if (vf_netdev)
>>  		__netvsc_vf_setup(ndev, vf_netdev);
>> @@ -1655,7 +1659,11 @@ static void netvsc_vf_update(struct work_struct *w)
>>  	struct net_device *vf_netdev;
>>  	bool vf_is_up;
>>
>> -	rtnl_lock();
>> +	if (!rtnl_trylock()) {
>> +		schedule_work(w);
>> +		return;
>> +	}
>> +
>
> So in the situation when we're currently in netvsc_unregister_vf() and
> trying to do
>         cancel_work_sync(&net_device_ctx->vf_takeover);
> 	cancel_work_sync(&net_device_ctx->vf_notify);
>
> we'll end up not executing netvsc_vf_update() at all, right? Wouldn't it
> create an issue as nobody is switching the datapath back to netvsc?
>

Actually, looking more at this I think we have additional issues:

netvsc_unregister_vf() may get executed _before_ netvsc_vf_update() gets
a chance and we just cancel it so the data path is never switched
back. I actually have a VM where I suppose it happens ...

[    7.235566] hv_netvsc 33b7a6f9-6736-451f-8fce-b382eaa50bee eth1: VF up: enP2p0s2
[    7.235569] hv_netvsc 33b7a6f9-6736-451f-8fce-b382eaa50bee eth1: Datapath switched to VF: enP2p0s2

On VF removal:

[   17.675885] mlx4_en: enP2p0s2: Close port called
[   17.727005] hv_netvsc 33b7a6f9-6736-451f-8fce-b382eaa50bee eth1: VF unregistering: enP2p0s2
<and nothing after - so the data path is not switched>

We need to make sure netvsc_vf_update() is always processed on removal.

-- 
  Vitaly