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

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

> > -----Original Message-----
> > From: Ferruh Yigit <ferruh.yigit@amd.com>
> > Sent: Saturday, March 4, 2023 1:19 AM
> > To: Konstantin Ananyev <konstantin.v.ananyev@yandex.ru>; dev@dpdk.org; fengchengwen
> > <fengchengwen@huawei.com>; Konstantin Ananyev <konstantin.ananyev@huawei.com>; Honnappa
> > Nagarahalli <Honnappa.Nagarahalli@arm.com>; Stephen Hemminger <stephen@networkplumber.org>;
> > Ruifeng Wang <Ruifeng.Wang@arm.com>; Ajit Khaparde (ajit.khaparde@broadcom.com)
> > <ajit.khaparde@broadcom.com>
> > Subject: Re: [PATCH 2/2] ethdev: fix race condition in fast-path ops setup
> >
> > 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?
> 
> Yes, updating only function pointers removes the synchronization requirement between function
> pointer and qdata.

Lads, that wouldn't work anyway.
The race between recovery procedure and data-path persists:
Recovery still has no idea is at given moment any thread doing RX/TX or not, and there is no
way for it to know when such thread will finish.
We do need some synchronization mechanism between control(recovery) and data-path threads.
I believe it is unavoidable.   

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