08/12/2017 13:31, 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?
If an entity steals ownership to another one, either it is agreed earlier, or it is done by a central authority. When it is acked that ownership can be moved, there should not be any configuration in progress. So it is more a communication issue than a race. > 2) Is device configuration racy with device addition/removal? That is to say, > can one thread remove a device while another configures it. I think it is the same as two threads configuring the same device (item 4/ above). It can be managed with port ownership. > 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?
