[dpdk-dev] [PATCH 2/5] ethdev: add port ownership

[Neil Horman]

On Fri, Dec 08, 2017 at 12:35:18PM +0100, Thomas Monjalon wrote:
> 05/12/2017 11:05, Bruce Richardson:
> > > I think you suggest to make all the ethdev configuration race safe, it
> > > is behind to this thread.  Current ethdev implementation leave the
> > > race management to applications, so port ownership as any other port
> > > configurations should be designed in the same method.
> > 
> > One key difference, though, being that port ownership itself could be
> > used to manage the thread-safety of the ethdev configuration. It's also
> > a little different from other APIs in that I find it hard to come up
> > with a scenario where it would be very useful to an application without
> > also having some form of locking present in it. For other config/control
> > APIs we can have the control plane APIs work without locks e.g. by
> > having a single designated thread/process manage all configuration
> > updates. However, as Neil points out, in such a scenario, the ownership
> > concept doesn't provide any additional benefit so can be skipped
> > completely. I'd view it a bit like the reference counting of mbufs -
> > we can provide a lockless/non-atomic version, but for just about every
> > real use-case, you want the atomic version.
> 
> I think we need to clearly describe what is the tread-safety policy
> in DPDK (especially in ethdev as a first example).
> Let's start with obvious things:
> 
> 	1/ A queue is not protected for races with multiple Rx or Tx
> 			- no planned change because of performance purpose
> 	2/ The list of devices is racy
> 			- to be fixed with atomics
> 	3/ The configuration of different devices is thread-safe
> 			- the configurations are different per-device
> 	4/ The configuration of a given device is racy
> 			- can be managed by the owner of the device
> 	5/ The device ownership is racy
> 			- to be fixed with atomics
> 
> What am I missing?
> 
There is fan out to consider here:

1) Is device configuration racy with ownership?  That is to say, can I change
ownership of a device safely while another thread that currently owns it
modifies its configuration?

2) Is device configuration racy with device addition/removal?  That is to say,
can one thread remove a device while another configures it.

There are probably many subsystem interactions that need to be addressed here.

Neil

> I am also wondering whether the device ownership can be a separate
> library used in several device class interfaces?
>