Re: [dpdk-dev] [PATCH v9 1/8] ethdev: use locks to protect Rx/Tx callback lists

[Ivan Boule <ivan.boule@6wind.com>]

On 06/15/2016 03:29 PM, Bruce Richardson wrote:
> On Wed, Jun 15, 2016 at 12:40:16PM +0000, Ananyev, Konstantin wrote:
>> Hi Ivan,
>>
>>> -----Original Message-----
>>> From: Ivan Boule [mailto:ivan.boule@6wind.com]
>>> Sent: Wednesday, June 15, 2016 1:15 PM
>>> To: Thomas Monjalon; Ananyev, Konstantin
>>> Cc: Pattan, Reshma; dev@dpdk.org
>>> Subject: Re: [dpdk-dev] [PATCH v9 1/8] ethdev: use locks to protect Rx/Tx callback lists
>>>
>>> On 06/15/2016 10:48 AM, Thomas Monjalon wrote:
>>>
>>>>>
>>>>>> I think the read access would need locking but we do not want it
>>>>>> in fast path.
>>>>>
>>>>> I don't think it would be needed.
>>>>> As I said - read/write interaction didn't change from what we have right now.
>>>>> But if you have some particular scenario in mind that you believe would cause
>>>>> a race condition - please speak up.
>>>>
>>>> If we add/remove a callback during a burst? Is it possible that the next
>>>> pointer would have a wrong value leading to a crash?
>>>> Maybe we need a comment to state that we should not alter burst
>>>> callbacks while running burst functions.
>>>>
>>>
>>> Hi Reshma,
>>>
>>> You claim that the "rte_eth_rx_cb_lock" does not need to be hold in the
>>> function "rte_eth_rx_burst()" while parsing the "post_rx_burst_cbs" list
>>> of RX callbacks associated with the polled RX queue to safely remove RX
>>> callback(s) in parallel.
>>> The problem is not [only] with the setting and the loading of "cb->next"
>>> that you assume to be atomic operations, which is certainly true on most
>>> CPUs.
>>> I see the 2 important following issues:
>>>
>>> 1) the "rte_eth_rxtx_callback" data structure associated with a removed
>>> RX callback could still be accessed in the callback parsing loop of the
>>> function "rte_eth_rx_burst()" after having been freed in parallel.
>>>
>>> BUT SUCH A BAD SITUATION WILL NOT CURRENTLY HAPPEN, THANKS TO THE NICE
>>> MEMORY LEAK BUG in the function "rte_eth_remove_rx_callback()"  that
>>> does not free the "rte_eth_rxtx_callback" data structure associated with
>>> the removed callback !
>>
>> Yes, though it is documented behaviour, someone can probably
>> refer it as a feature, not a bug ;)
>>
>
> +1
> This is definitely not a bug, this is absolutely by design. One may argue with
> the design, but it was done for a definite reason, so as to avoid paying the
> penalty of having locks. It pushes more responsibility onto the app, but it
> does allow the app to choose the best solution for managing the freeing of
> memory for its situation. The alternative is to force all apps to pay the cost
> of having locks, even if better options for freeing the memory are available.
>
> /Bruce
>

-1 (not to say 0xFFFFFFFF)

This is definitely an API design bug !
I would say that if you don't know how to free a resource that you 
allocate, it is very likely that you are wrong allocating it.
And this is exactly what happens here with RX/TX callback data structures.
This problem can easily be addressed by just changing the API as follows:

Change
     void *
     rte_eth_add_rx_callback(uint8_t port_id, uint16_t queue_id,
                             rte_rx_callback_fn fn, void *user_param)

to
     int
     rte_eth_add_rx_callback(uint8_t port_id, uint16_t queue_id,
                             struct rte_eth_rxtx_callback *cb)

In addition of solving the problem, this approach makes the API 
consistent and let the application allocate "rte_eth_rxtx_callback" data 
structures through any appropriate mean.

Regards,
Ivan

-- 
Ivan Boule
6WIND Development Engineer