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

[Ajit Khaparde <ajit.khaparde@broadcom.com>]

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On Mon, Mar 6, 2023 at 2:33 AM Konstantin Ananyev
<konstantin.ananyev@huawei.com> wrote:
>
>
>
> > > -----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.
+1

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

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