On 09/19/2018 04:07 PM, Cornelia Huck wrote: >> Yep, I'm for doing something like this first. We can think about doing >> something more elaborate later. I will send a non-RFC with an extra >> per-device mutex. Unless you object. > No, that sounds good to me. > OK. >> Another thought that crossed my head was making the transport ops >> mutex on each virtio-ccw device -- see our conversation on get/set >> config. I don't think it would make a big difference, since the >> ccw stuff is mutex already, so we only have parallelism for the >> preparation and for post-processing the results of the ccw io. > Do you spot any other places where we may need to care about concurrent > processing (like for the ->config area in the previous patch)? > It is hard to tell, because: * Synchronization external to the transport could make things work out just fine. * virtio_config_ops does not document these requirements if any. * So it's up to the devices to use the stuff without shooting themselves in the foot. * virtio-pci does not seem to do more to avoid such problems that we do. Back then when learning vritio-ccw I did ask myself such questions and based on vrito-pci and I was like looks similar, should be good enough. But I guess you know all this and that's why you said "any other places where we *may* need to care". Some of the places I'm not sure we don't are: * status get/set, and * find_vqs, del_vqs But if I had been sure that these are problematic, I would have pointed that out. Status is one byte, and that could save the day. But I can't recall if the architecture guarantees that we won't observe anything funny. Regarding find_vqs I think of some external-ish events that could make us tear down before build-up (find_vqs) finished. Funny thing is we do protect the list with a lock, but the lock is taken only while dealing with a single element. So I don't think it would save us. I hope synchronization external to virtio-ccw takes care of this, but I cant tell for sure. I'm only sure that I don't fully understand the interactions. I stated that in the cover letter ;). Regards, Halil
