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

[Konstantin Ananyev <konstantin.ananyev@huawei.com>]

> >>>>>>> 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.
And right now rte_ethdev layer doesn't provide any mechanism to check it or wait when they'll finish, etc.

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
  (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.

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