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

[fengchengwen <fengchengwen@huawei.com>]

On 2023/2/23 21:31, 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.

> 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.

> 
> 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.

> 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;
>>>>>>  }
>>>>>>
>>>>>>
>>>>>> .
>>>>>>
>>>