netdev - Re: [PATCH net] hv_netvsc: Fix race between VF offering and VF association message from host

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Date:   Thu, 6 Oct 2022 09:43:30 +0530
From:   Gaurav Kohli <gauravkohli@...ux.microsoft.com>
To:     Haiyang Zhang <haiyangz@...rosoft.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     KY Srinivasan <kys@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net] hv_netvsc: Fix race between VF offering and VF
 association message from host


On 9/30/2022 6:33 PM, Haiyang Zhang wrote:
>
>> -----Original Message-----
>> From: Jakub Kicinski <kuba@...nel.org>
>> Sent: Thursday, September 29, 2022 10:26 PM
>> To: Gaurav Kohli <gauravkohli@...ux.microsoft.com>
>> Cc: KY Srinivasan <kys@...rosoft.com>; Haiyang Zhang
>> <haiyangz@...rosoft.com>; Stephen Hemminger
>> <sthemmin@...rosoft.com>; wei.liu@...nel.org; Dexuan Cui
>> <decui@...rosoft.com>; linux-hyperv@...r.kernel.org;
>> netdev@...r.kernel.org
>> Subject: Re: [PATCH net] hv_netvsc: Fix race between VF offering and VF
>> association message from host
>>
>> On Wed, 28 Sep 2022 06:48:33 -0700 Gaurav Kohli wrote:
>>> During vm boot, there might be possibility that vf registration
>>> call comes before the vf association from host to vm.
>>>
>>> And this might break netvsc vf path, To prevent the same block
>>> vf registration until vf bind message comes from host.
>>>
>>> Cc: stable@...r.kernel.org
>>> Fixes: 00d7ddba11436 ("hv_netvsc: pair VF based on serial number")
>>> Signed-off-by: Gaurav Kohli <gauravkohli@...ux.microsoft.com>
>> Is it possible to add a timeout or such? Waiting for an external
>> event while holding rtnl lock seems a little scary.
> We used to have time-out in many places of this driver. But there is
> no protocol guarantees of the host response time, so the time out value
> cannot be set. These time-outs were removed several years ago.
>
>
>> The other question is - what protects the completion and ->vf_alloc
>> from races? Is there some locking? ->vf_alloc only goes from 0 to 1
>> and never back?

Thanks for the comment, i understand your concern for vf_alloc and 
reinit completion part, I think

we can move reinit completion to unregistration part of vf code.

Let me send v2 patch.

> When Vf is removed, the vf_assoc msg will set it to 0 here:
>          net_device_ctx->vf_alloc = nvmsg->msg.v4_msg.vf_assoc.allocated;
>          net_device_ctx->vf_serial = nvmsg->msg.v4_msg.vf_assoc.serial;
>
> Also, I think this condition can be changed from:
> +	if (vf_is_up && !net_device_ctx->vf_alloc) {

Thanks for the comment.

This is needed to maintain state machine, as netvsc change event can 
comes multiple time. That's why i have put

extra check to avoid any deadlock.

> to:
> +	if (vf_is_up) {
> So when VF comes up, it always wait for the completion without depending
> on the vf_alloc.
>
> Thanks,
> - Haiyang