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

[Stephen Hemminger <stephen@networkplumber.org>]

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.

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.