Re: [PATCH v3 2/5] bus/vdev: add lock on vdev device list

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 20-Apr-18 3:19 PM, Tan, Jianfeng wrote:
> 
> 
> On 4/20/2018 4:26 PM, Burakov, Anatoly wrote:
>> On 19-Apr-18 5:50 PM, Jianfeng Tan wrote:
>>> As we could add virtual devices from different threads now, we
>>> add a spin lock to protect the vdev device list.
>>>
>>> Suggested-by: Anatoly Burakov <anatoly.burakov@intel.com>
>>> Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
>>> Reviewed-by: Qi Zhang <qi.z.zhang@intel.com>
>>> ---
>>
>> <...>
>>
>>> +/* The caller shall be responsible for thread-safe */
>>>   static struct rte_vdev_device *
>>>   find_vdev(const char *name)
>>>   {
>>> @@ -203,10 +206,6 @@ rte_vdev_init(const char *name, const char *args)
>>>       if (name == NULL)
>>>           return -EINVAL;
>>>   -    dev = find_vdev(name);
>>> -    if (dev)
>>> -        return -EEXIST;
>>> -
>>>       devargs = alloc_devargs(name, args);
>>>       if (!devargs)
>>>           return -ENOMEM;
>>> @@ -221,16 +220,28 @@ rte_vdev_init(const char *name, const char *args)
>>>       dev->device.numa_node = SOCKET_ID_ANY;
>>>       dev->device.name = devargs->name;
>>>   +    rte_spinlock_lock(&vdev_device_list_lock);
>>> +    if (find_vdev(name)) {
>>> +        rte_spinlock_unlock(&vdev_device_list_lock);
>>> +        ret = -EEXIST;
>>> +        goto fail;
>>> +    }
>>> +    TAILQ_INSERT_TAIL(&vdev_device_list, dev, next);
>>> +    rte_spinlock_unlock(&vdev_device_list_lock);
>>> +
>>
>> I wonder if is possible to just leave the tailq locked until you 
>> either insert the device into tailq, or figure out that it's not 
>> possible? Seems like doing two locks here is unnecessary, unless 
>> vdev_probe_all_drivers needs this tailq unlocked...
> 
> My opinion is that we don't know what could be done in driver probe(). 
> It could possibly insert a new vdev (it does not happen now, but could 
> happen in future?). So here, we call this with tailq unlocked. Or we 
> keep it as simple as possible as you say?

I thought this code was responsible for inserting vdevs? I think it 
would be generally bad design to insert vdev while inserting vdev :)

That said, it's a fair point, and i don't have a strong opinion on this, 
so you can leave it as is if you want.

-- 
Thanks,
Anatoly