netdev - Re: [Patch v9 03/12] net: mana: Handle vport sharing between devices

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Date:   Tue, 25 Oct 2022 09:08:34 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Long Li <longli@...rosoft.com>, KY Srinivasan <kys@...rosoft.com>,
        "Haiyang Zhang" <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Leon Romanovsky <leon@...nel.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "shiraz.saleem@...el.com" <shiraz.saleem@...el.com>,
        Ajay Sharma <sharmaajay@...rosoft.com>
CC:     "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
Subject: Re: [Patch v9 03/12] net: mana: Handle vport sharing between devices

On 2022/10/25 2:45, Long Li wrote:
>>> +int mana_cfg_vport(struct mana_port_context *apc, u32
>> protection_dom_id,
>>> +		   u32 doorbell_pg_id)
>>>  {
>>>  	struct mana_config_vport_resp resp = {};
>>>  	struct mana_config_vport_req req = {};
>>>  	int err;
>>>
>>> +	/* This function is used to program the Ethernet port in the hardware
>>> +	 * table. It can be called from the Ethernet driver or the RDMA driver.
>>> +	 *
>>> +	 * For Ethernet usage, the hardware supports only one active user on
>> a
>>> +	 * physical port. The driver checks on the port usage before
>> programming
>>> +	 * the hardware when creating the RAW QP (RDMA driver) or
>> exposing the
>>> +	 * device to kernel NET layer (Ethernet driver).
>>> +	 *
>>> +	 * Because the RDMA driver doesn't know in advance which QP type
>> the
>>> +	 * user will create, it exposes the device with all its ports. The user
>>> +	 * may not be able to create RAW QP on a port if this port is already
>>> +	 * in used by the Ethernet driver from the kernel.
>>> +	 *
>>> +	 * This physical port limitation only applies to the RAW QP. For RC QP,
>>> +	 * the hardware doesn't have this limitation. The user can create RC
>>> +	 * QPs on a physical port up to the hardware limits independent of
>> the
>>> +	 * Ethernet usage on the same port.
>>> +	 */
>>> +	mutex_lock(&apc->vport_mutex);
>>> +	if (apc->vport_use_count > 0) {
>>> +		mutex_unlock(&apc->vport_mutex);
>>> +		return -EBUSY;
>>> +	}
>>> +	apc->vport_use_count++;
>>> +	mutex_unlock(&apc->vport_mutex);
>>> +
>>>  	mana_gd_init_req_hdr(&req.hdr, MANA_CONFIG_VPORT_TX,
>>>  			     sizeof(req), sizeof(resp));
>>>  	req.vport = apc->port_handle;
>>> @@ -679,9 +714,16 @@ static int mana_cfg_vport(struct
>>> mana_port_context *apc, u32 protection_dom_id,
>>>
>>>  	apc->tx_shortform_allowed = resp.short_form_allowed;
>>>  	apc->tx_vp_offset = resp.tx_vport_offset;
>>> +
>>> +	netdev_info(apc->ndev, "Configured vPort %llu PD %u DB %u\n",
>>> +		    apc->port_handle, protection_dom_id, doorbell_pg_id);
>>>  out:
>>> +	if (err)
>>> +		mana_uncfg_vport(apc);
>>
>> There seems to be a similar race between error handling here and the "apc-
>>> vport_use_count > 0" checking above as pointed out in v7.
> 
> Thanks for looking into this.
> 
> This is different to the locking bug in mana_ib_cfg_vport(). The vport sharing
> between Ethernet and RDMA is exclusive, not shared. If another driver tries
> to take the vport while it is being configured, it will fail immediately. It is by

Suppose the following steps:
1. Ethernet driver take the lock first and do a "apc->vport_use_count++", and
   release the lock;
2. RDMA driver take the lock, do "apc->vport_use_count > 0" checking and return
   -EBUSY;
3. mana_send_request() or mana_verify_resp_hdr() return error to Ethernet driver.

It seems that vport is left unused when above happens, if that is what you wanted?


> design to prevent possible deadlock.

I am not sure I understand the deadlock here.

>