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From: Konstantin Ananyev <konstantin.ananyev@huawei.com>
To: Stephen Hemminger <stephen@networkplumber.org>,
	Luc Pelletier <lucp.at.work@gmail.com>
Cc: "grive@u256.net" <grive@u256.net>, "dev@dpdk.org" <dev@dpdk.org>,
	"stable@dpdk.org" <stable@dpdk.org>,
	"hyonkim@cisco.com" <hyonkim@cisco.com>,
	 "johndale@cisco.com" <johndale@cisco.com>
Subject: RE: [PATCH] failsafe: fix segfault on hotplug event
Date: Fri, 18 Nov 2022 16:31:34 +0000	[thread overview]
Message-ID: <774190f3094342c28de2e28f9d93d486@huawei.com> (raw)
In-Reply-To: <20221116142548.554a7a9e@hermes.local>



Hi Luc,
 
> > Hi Konstantin,
> >
> > > It is not recommended way to update rte_eth_fp_ops[] contents directly.
> > > There are eth_dev_fp_ops_setup()/ eth_dev_fp_ops_reset() that supposed
> > > to be used for that.
> >
> > Good to know. I see another fix that was made in a different PMD that
> > does exactly the same thing:
> >
> > https://github.com/DPDK/dpdk/commit/bcd68b68415172815e55fc67cf3947c0433baf74
> >
> > CC'ing the authors for awareness.
> >
> > > About the fix itself - while it might help till some extent,
> > > I think it will not remove the problem completely.
> > > There still remain a race-condition between rte_eth_rx_burst() and failsafe_eth_rmv_event_callback().
> > > Right now DPDK doesn't support switching PMD fast-ops functions (or updating rxq/txq data)
> > > on the fly.
> >
> > Thanks for the information. This is very helpful.
> >
> > Are you saying that the previous code also had that same race
> > condition?

Yes, I believe so. 

> It was only updating the rte_eth_dev structure, but I
> > assume the problem would have been the same since rte_eth_rx_burst()
> > in DPDK versions <=20 use the function pointers in rte_eth_dev, not
> > rte_eth_fp_ops.
> >
> > Can you think of a possible solution to this problem? I'm happy to
> > provide a patch to properly fix the problem. Having your guidance
> > would be extremely helpful.
> >
> > Thanks!
> 
> Changing burst mode on a running device is not safe because
> of lack of locking and/or memory barriers.
> 
> Would have been better to not to do this optimization.
> Just have one rx_burst/tx_burst function and look at what
> ever conditions are present there.

I think Stephen is right - within DPDK it is just not possible to switch RX/TX
function on the fly (without some external synchronization).
So the safe way is to always use safe version of RX/TX call.
I personally don't think such few extra checks will affect performance that much.

As another nit: inside failsafe rx_burst functions it is probably better not to access dev->data->rx_queues directly,
but call rte_eth_rx_burst(sdev->sdev_port_id, ...); instead.
Same for TX.

Thanks
Konstantin

 



  reply	other threads:[~2022-11-18 16:31 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-11-10 16:34 Luc Pelletier
2022-11-10 17:42 ` [PATCH v2] " Luc Pelletier
2022-11-10 23:33   ` Stephen Hemminger
2022-11-16 17:35 ` [PATCH] " Konstantin Ananyev
2022-11-16 21:51   ` Luc Pelletier
2022-11-16 22:25     ` Stephen Hemminger
2022-11-18 16:31       ` Konstantin Ananyev [this message]
2022-11-29 14:48 ` [PATCH v3 1/5] " Luc Pelletier
2023-10-31 17:35   ` Stephen Hemminger
2022-11-29 14:48 ` [PATCH v3 3/5] failsafe: fix double release of port Luc Pelletier
2022-11-29 14:48 ` [PATCH v3 4/5] failsafe: use public APIs in fs_link_update Luc Pelletier
2022-11-29 14:48 ` [PATCH v3 5/5] failsafe: make sub-device remove flag thread-safe Luc Pelletier
2022-11-29 15:25 ` [PATCH v4 1/5] failsafe: fix segfault on hotplug event Luc Pelletier
2022-11-29 15:25 ` [PATCH v4 3/5] failsafe: fix double release of port Luc Pelletier
2022-11-29 15:25 ` [PATCH v4 4/5] failsafe: use public APIs in fs_link_update Luc Pelletier
2022-11-29 15:25 ` [PATCH v4 5/5] failsafe: make sub-device remove flag thread-safe Luc Pelletier

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