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

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

> -----Original Message-----
> From: Stephen Hemminger <stephen@networkplumber.org>
> Sent: Wednesday, February 22, 2023 7:15 PM
> To: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
> Cc: Konstantin Ananyev <konstantin.ananyev@huawei.com>; Fengchengwen
> <fengchengwen@huawei.com>; Ruifeng Wang <Ruifeng.Wang@arm.com>;
> Ashok Kaladi <ashok.k.kaladi@intel.com>; jerinj@marvell.com;
> thomas@monjalon.net; dev@dpdk.org; s.v.naga.harish.k@intel.com;
> erik.g.carrillo@intel.com; abhinandan.gujjar@intel.com; stable@dpdk.org; nd
> <nd@arm.com>
> Subject: Re: [PATCH 2/2] ethdev: fix race condition in fast-path ops setup
> 
> On Wed, 22 Feb 2023 22:48:25 +0000
> Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com> wrote:
> 
> > > -----Original Message-----
> > > From: Konstantin Ananyev <konstantin.ananyev@huawei.com>
> > > Sent: Wednesday, February 22, 2023 4:41 AM
> > > To: Fengchengwen <fengchengwen@huawei.com>; Stephen Hemminger
> > > <stephen@networkplumber.org>; Ruifeng Wang
> <Ruifeng.Wang@arm.com>
> > > Cc: Ashok Kaladi <ashok.k.kaladi@intel.com>; jerinj@marvell.com;
> > > thomas@monjalon.net; Honnappa Nagarahalli
> > > <Honnappa.Nagarahalli@arm.com>; dev@dpdk.org;
> > > s.v.naga.harish.k@intel.com; erik.g.carrillo@intel.com;
> > > abhinandan.gujjar@intel.com; stable@dpdk.org; nd <nd@arm.com>
> > > Subject: RE: [PATCH 2/2] ethdev: fix race condition in fast-path ops
> > > setup
> > >
> > >
> > >
> > > > -----Original Message-----
> > > > From: fengchengwen <fengchengwen@huawei.com>
> > > > Sent: Wednesday, February 22, 2023 1:07 AM
> > > > To: Stephen Hemminger <stephen@networkplumber.org>; Ruifeng
> Wang
> > > > <Ruifeng.Wang@arm.com>
> > > > Cc: Ashok Kaladi <ashok.k.kaladi@intel.com>; jerinj@marvell.com;
> > > > thomas@monjalon.net; Honnappa Nagarahalli
> > > > <Honnappa.Nagarahalli@arm.com>; dev@dpdk.org;
> > > > s.v.naga.harish.k@intel.com; erik.g.carrillo@intel.com;
> > > > abhinandan.gujjar@intel.com; stable@dpdk.org; nd <nd@arm.com>
> > > > Subject: Re: [PATCH 2/2] ethdev: fix race condition in fast-path
> > > > ops setup
> > > >
> > > > On 2023/2/22 1:00, Stephen Hemminger wrote:
> > > > > On Tue, 21 Feb 2023 07:24:19 +0000 Ruifeng Wang
> > > > > <Ruifeng.Wang@arm.com> wrote:
> > > > >
> > > > >>> -----Original Message-----
> > > > >>> From: fengchengwen <fengchengwen@huawei.com>
> > > > >>> Sent: Monday, February 20, 2023 2:58 PM
> > > > >>> To: Ashok Kaladi <ashok.k.kaladi@intel.com>;
> > > > >>> jerinj@marvell.com; thomas@monjalon.net
> > > > >>> Cc: dev@dpdk.org; s.v.naga.harish.k@intel.com;
> > > > >>> erik.g.carrillo@intel.com; abhinandan.gujjar@intel.com;
> > > > >>> stable@dpdk.org; Ruifeng Wang <Ruifeng.Wang@arm.com>
> > > > >>> Subject: Re: [PATCH 2/2] ethdev: fix race condition in
> > > > >>> fast-path ops setup
> > > > >>>
> > > > >>> On 2023/2/20 14:08, Ashok Kaladi 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.
> > +1
> > Any such synchronization typically requires using load-acquire on data
> plane, which brings down the performance. But, init time synchronization
> would not affect the performance (stating the obvious).
> >
> > > 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...
> > > It probably needs to be either deprecated or reworked.
> > >
> > > >
> > > > >
> > > > > 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;
> > > > >  }
> > > > >
> > > > >
> > > > > .
> > > > >
> >
> 
> What about making started a real flag (with weak atomic's) and then any
> dataplane threads should wait for started flag before going into main loop.
This does not solve the later loads getting hoisted before reading the 'flag'.

> 
> It would not solve the error recovery case where the device decides to take
> itself offline. But that design is racy to start with and needs to be redesigned.