Re: [PATCH 2/2] ethdev: fix race condition in fast-path ops setup

[fengchengwen <fengchengwen@huawei.com>]

On 2023/3/4 1:19, Ferruh Yigit wrote:
> On 2/26/2023 5:22 PM, Konstantin Ananyev wrote:
>>
>>>>>>>>>>>> If ethdev enqueue or dequeue function is called during
>>>>>>>>>>>> eth_dev_fp_ops_setup(), it may get pre-empted after setting the
>>>>>>>>>>>> function pointers, but before setting the pointer to port data.
>>>>>>>>>>>> In this case the newly registered enqueue/dequeue function will
>>>>>>>>>>>> use dummy port data and end up in seg fault.
>>>>>>>>>>>>
>>>>>>>>>>>> This patch moves the updation of each data pointers before
>>>>>>>>>>>> updating corresponding function pointers.
>>>>>>>>>>>>
>>>>>>>>>>>> Fixes: c87d435a4d79 ("ethdev: copy fast-path API into separate
>>>>>>>>>>>> structure")
>>>>>>>>>>>> Cc: stable@dpdk.org
>>>>>>>>>
>>>>>>>>> Why is something calling enqueue/dequeue when device is not fully
>>>>>>> started.
>>>>>>>>> A correctly written application would not call rx/tx burst until
>>>>>>>>> after ethdev start had finished.
>>>>>>>>
>>>>>>>> Please refer the eb0d471a894 (ethdev: add proactive error handling
>>>>>>>> mode), when driver recover itself, the application may still invoke
>>>>>>> enqueue/dequeue API.
>>>>>>>
>>>>>>> Right now DPDK ethdev layer *does not* provide synchronization
>>>>>>> mechanisms between data-path and control-path functions.
>>>>>>> That was a deliberate deisgn choice. If we want to change that
>>>>>>> rule, then I
>>>>>>> suppose we need a community consensus for it.
>>>>>>> I think that if the driver wants to provide some sort of error
>>>>>>> recovery
>>>>>>> procedure, then it has to provide some synchronization mechanism
>>>>>>> inside it
>>>>>>> between data-path and control-path functions.
>>>>>>> Actually looking at eb0d471a894 (ethdev: add proactive error handling
>>>>>>> mode), and following patches I wonder how it creeped in?
>>>>>>> It seems we just introduced a loophole for race condition with this
>>>>>>> approach...
>>>>>
>>>>> Could you try to describe the specific scenario of loophole ?
>>>>
>>>> Ok, as I understand the existing mechanism:
>>>>
>>>> When PMD wants to start a recovery it has to:
>>>>   - invoke  rte_eth_dev_callback_process(RTE_ETH_EVENT_ERR_RECOVERING);
>>>>     That supposed to call user provided callback. After callback is
>>>> finished PMD assumes
>>>>     that user is aware that recovery is about to start and should
>>>> make some precautions.
>>>> - when recovery is finished it invokes another callback:
>>>>    RTE_ETH_EVENT_RECOVERY_(SUCCESS/FAILED). After that user either
>>>> can continue to
>>>>    use port or have to treat is as faulty.
>>>>
>>>> The idea is ok in principle, but there is a problem.
>>>>
>>>> lib/ethdev/rte_ethdev.h:
>>>>             /** Port recovering from a hardware or firmware error.
>>>>           * If PMD supports proactive error recovery,
>>>>           * it should trigger this event to notify application
>>>>           * that it detected an error and the recovery is being started.
>>>>
>>>> <<< !!!!!
>>>>           * Upon receiving the event, the application should not
>>>> invoke any control path API
>>>>           * (such as rte_eth_dev_configure/rte_eth_dev_stop...) until
>>>> receiving
>>>>           * RTE_ETH_EVENT_RECOVERY_SUCCESS or
>>>> RTE_ETH_EVENT_RECOVERY_FAILED event.
>>>>           * The PMD will set the data path pointers to dummy functions,
>>>>           * and re-set the data path pointers to non-dummy functions
>>>>           * before reporting RTE_ETH_EVENT_RECOVERY_SUCCESS event.
>>>> <<< !!!!!
>>>>
>>>> That part is just wrong I believe.
>>>> It should be:
>>>> Upon receiving the event, the application should not invoke any *both
>>>> control and data-path* API
>>>> until receiving  RTE_ETH_EVENT_RECOVERY_SUCCESS or
>>>> RTE_ETH_EVENT_RECOVERY_FAILED event.
>>>> Resetting data path pointers to dummy functions by PMD *before* invoking
>>>> rte_eth_dev_callback_process(RTE_ETH_EVENT_ERR_RECOVERING);
>>>> introduces a race-condition with data-path threads, as such thread
>>>> could already be inside RX/TX function
>>>> or can already read RX/TX function/data pointers and be about to use
>>>> them.
>>>
>>> Current practices: the PMDs already add some delay after set Rx/Tx
>>> callback to dummy, and plus the DPDK
>>> worker thread is busypolling, the probability of occurence in reality
>>> is zero. But in theoretically exist
>>> the above race-condition.
>>
>>
>> Adding delay might make a problem a bit less reproducible,
>> but it doesn't fix it.
>> The bug is still there.
>>
>>
>>>
>>>> And right now rte_ethdev layer doesn't provide any mechanism to check
>>>> it or wait when they'll finish, etc.
>>>
>>> Yes
>>>
>>>>
>>>> So, probably the simplest way to fix it with existing DPDK design:
>>>> - user level callback  RTE_ETH_EVENT_ERR_RECOVERING should return
>>>> only after it ensures that *all*
>>>>    application threads (and processes) stopped using either control
>>>> or data-path functions for that port
>>>
>>> Agree
>>>
>>>>    (yes it means that application that wants to use this feature has
>>>> to provide its own synchronization mechanism
>>>>    around data-path functions (RX/TX) that it is going to use).
>>>> - after that PMD is safe to reset rte_eth_fp_ops[] values to dummy ones.
>>>>
>>>> And message to all PMD developers:
>>>> *please stop updating rte_eth_fp_ops[] on your own*.
>>>> That's a bad practice and it is not supposed to do things that way.
>>>> There is a special API provided for these purposes:
>>>> eth_dev_fp_ops_reset(), eth_dev_fp_ops_setup(), so use it.
>>>
>>> This two function is in private.h, so it should be expose to public
>>> header file.
>>
>> You mean we need to move these functions declarations into ethdev_driver.h?
>> If so, then yes, I think we probably do.
>>
>>
> 
> 
> What about making slightly different version available to drivers, which
> only updates function pointers, but not  'fpo->rxq' / 'fpo->txq'.
> 
> This way driver can switch to between dummy and real burst function
> without worrying Rx/Tx queue validity.
> 
> @Chengwen, @Ruifeng, can this solve the issue for relaxed memory
> ordering systems?

For the problem described in this commit, I think it's OK for solve the RMO.

> 
> 
> 
>>>>
>>>> BTW,  I don't see any implementation for RTE_ETH_EVENT_ERR_RECOVERING
>>>> within
>>>> either testpmd or any other example apps.
>>>> Am I missing something?
>>>
>>> Currently it just promote the event.
>>
>>
>> Ok, can I suggest then to add a proper usage for into in testpmd?
>> It looks really strange that we add new feature into ethdev (and 2 PMDs),
>> but didn't provide any way for users to test it.
>>
>>>
>>>> If not, then probably it could be a good starting point - let's
>>>> incorporate it inside testpmd
>>>> (new forwarding engine probably) so everyone can test/try it.
>>>>
>>>>           * It means that the application cannot send or receive any
>>>> packets
>>>>           * during this period.
>>>>           * @note Before the PMD reports the recovery result,
>>>>           * the PMD may report the RTE_ETH_EVENT_ERR_RECOVERING event
>>>> again,
>>>>           * because a larger error may occur during the recovery.
>>>>           */
>>>>          RTE_ETH_EVENT_ERR_RECOVERING,
>>>>
>>>>>>> It probably needs to be either deprecated or reworked.
>>>>>> Looking at the commit, it does not say anything about the data
>>>>>> plane functions which probably means, the error recovery is
>>>>> happening within the data plane thread. What happens to other data
>>>>> plane threads that are polling the same port on which the error
>>>>> recovery is happening?
>>>>>
>>>>> The commit log says: "the PMD sets the data path pointers to dummy
>>>>> functions".
>>>>>
>>>>> So the data plane threads will receive non-packet and send zero with
>>>>> port which in error recovery.
>>>>>
>>>>>>
>>>>>> Also, the commit log says that while the error recovery is under
>>>>>> progress, the application should not call any control plane APIs. Does
>>>>> that mean, the application has to check for error condition every
>>>>> time it calls a control plane API?
>>>>>
>>>>> If application has not register event (RTE_ETH_EVENT_ERR_RECOVERING)
>>>>> callback, it could calls control plane API, but it will return
>>>>> failed.
>>>>> If application has register above callback, it can wait for recovery
>>>>> result, or direct call without wait but this will return failed.
>>>>>
>>>>>>
>>>>>> The commit message also says that "PMD makes sure the control path
>>>>>> operations failed with retcode -EBUSY". It does not say how it
>>>>> does this. But, any communication from the PMD thread to control
>>>>> plane thread may introduce race conditions if not done correctly.
>>>>>
>>>>> First there are no PMD thread, do you mean eal-intr-thread ?
>>>>>
>>>>> As for this question, you can see PMDs which already implement it,
>>>>> they both provides mutual exclusion protection.
>>>>>
>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>> Would something like this work better?
>>>>>>>>>
>>>>>>>>> Note: there is another bug in current code. The check for link
>>>>>>>>> state
>>>>>>>>> interrupt and link_ops could return -ENOTSUP and leave device in
>>>>>>> indeterminate state.
>>>>>>>>> The check should be done before calling PMD.
>>>>>>>>>
>>>>>>>>> diff --git a/lib/ethdev/rte_ethdev.c b/lib/ethdev/rte_ethdev.c
>>>>>>>>> index
>>>>>>>>> 0266cc82acb6..d6c163ed85e7 100644
>>>>>>>>> --- a/lib/ethdev/rte_ethdev.c
>>>>>>>>> +++ b/lib/ethdev/rte_ethdev.c
>>>>>>>>> @@ -1582,6 +1582,14 @@ rte_eth_dev_start(uint16_t port_id)
>>>>>>>>>           return 0;
>>>>>>>>>       }
>>>>>>>>>
>>>>>>>>> +    if (dev->data->dev_conf.intr_conf.lsc == 0 &&
>>>>>>>>> +        dev->dev_ops->link_update == NULL) {
>>>>>>>>> +        RTE_ETHDEV_LOG(INFO,
>>>>>>>>> +                   "Device with port_id=%"PRIu16" link update not
>>>>>>> supported\n",
>>>>>>>>> +                   port_id);
>>>>>>>>> +            return -ENOTSUP;
>>>>>>>>> +    }
>>>>>>>>> +
>>>>>>>>>       ret = rte_eth_dev_info_get(port_id, &dev_info);
>>>>>>>>>       if (ret != 0)
>>>>>>>>>           return ret;
>>>>>>>>> @@ -1591,9 +1599,7 @@ rte_eth_dev_start(uint16_t port_id)
>>>>>>>>>           eth_dev_mac_restore(dev, &dev_info);
>>>>>>>>>
>>>>>>>>>       diag = (*dev->dev_ops->dev_start)(dev);
>>>>>>>>> -    if (diag == 0)
>>>>>>>>> -        dev->data->dev_started = 1;
>>>>>>>>> -    else
>>>>>>>>> +    if (diag != 0)
>>>>>>>>>           return eth_err(port_id, diag);
>>>>>>>>>
>>>>>>>>>       ret = eth_dev_config_restore(dev, &dev_info, port_id); @@
>>>>>>>>> -1611,16
>>>>>>>>> +1617,18 @@ rte_eth_dev_start(uint16_t port_id)
>>>>>>>>>           return ret;
>>>>>>>>>       }
>>>>>>>>>
>>>>>>>>> -    if (dev->data->dev_conf.intr_conf.lsc == 0) {
>>>>>>>>> -        if (*dev->dev_ops->link_update == NULL)
>>>>>>>>> -            return -ENOTSUP;
>>>>>>>>> -        (*dev->dev_ops->link_update)(dev, 0);
>>>>>>>>> -    }
>>>>>>>>> -
>>>>>>>>>       /* expose selection of PMD fast-path functions */
>>>>>>>>>       eth_dev_fp_ops_setup(rte_eth_fp_ops + port_id, dev);
>>>>>>>>>
>>>>>>>>> +    /* ensure state is set before marking device ready */
>>>>>>>>> +    rte_smp_wmb();
>>>>>>>>> +
>>>>>>>>>       rte_ethdev_trace_start(port_id);
>>>>>>>>> +
>>>>>>>>> +    /* Update current link state */
>>>>>>>>> +    if (dev->data->dev_conf.intr_conf.lsc == 0)
>>>>>>>>> +        (*dev->dev_ops->link_update)(dev, 0);
>>>>>>>>> +
>>>>>>>>>       return 0;
>>>>>>>>>   }
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> .
>>>>>>>>>
>>>>>>
>>
> 
> .
>