Re: [dpdk-dev] [PATCH] bus/vdev: fix probe same device twice

["Gaëtan Rivet" <gaetan.rivet@6wind.com>]

On Tue, Nov 06, 2018 at 09:36:22PM +0100, Thomas Monjalon wrote:
> 06/11/2018 16:46, Zhang, Qi Z:
> > From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > > 
> > > Hi,
> > > 
> > > 06/11/2018 01:31, Qi Zhang:
> > > > When probe the same device at second time
> > > 
> > > Sorry I stop on this first sentence.
> > > How and why do you probe a vdev twice?
> > 
> > if we do rte_dev_hotplug_add or rte_dev_proble on a probed device. (yes, this is not usually what an application want, but it can happen by miss-operation, and this is covered by our test case, it make sense to me that hotplug API should be robust enough to handle that situation.)
> 
> Yes I agree we must handle this situation.
> 
> > we will failed at the second time as expected, 
> > but will not able to detach the device any more, since during the second scan, original vdev->device.devargs is corrupted.
> 
> The root cause is we remove a devargs which was referenced.
> Could we overwrite the first devargs instead of removing it?
> 
> 

It's also possible to add a back-reference to an rte_device in [1],
but that can only work if only one rte_device references a devargs.
It seems to be the case now, but it might be good to enforce explicitly
that when a bus scans its devices, it should do a 1-to-1 map to devargs.

If mapping rte_device to rte_devargs needs to respect rules, it could
help bus developpers to have a function that will do the job: verify that
the devargs is not currently used, add the back-reference to the
rte_device.

With the proper back-reference, it is possible to clean-up the device
when removing the devargs (and also to add the rte_devargs_extract()
function that would allow keeping the original devargs and insert it back
if the hotplug fails, then the mapping must be restored).

[1]: https://mails.dpdk.org/archives/dev/2018-November/118274.html

-- 
Gaëtan Rivet
6WIND