On Sunday, December 30, 2018 2:06 PM, Halil Pasic wrote: > > I guess you are the first one trying to read virtio config from within interrupt > context. AFAICT this never worked. I'm not sure about "never worked". It seems to work well with virtio-pci. But looking forward to hearing a solid reason why reading config inside the handler is forbidden (if that's true). > About what happens. The apidoc of ccw_device_start() says it needs to be > called with the ccw device lock held, so ccw_io_helper() tries to take it (since > forever I guess). OTOH do_cio_interrupt() takes the subchannel lock and > io_subchannel_initialize_dev() makes the ccw device lock be the subchannel > lock. That means when one tries to get virtio config form within a cio > interrupt context we deadlock, because we try to take a lock we already have. > > That said, I don't think this limitation is by design (i.e. intended). > Maybe Connie can help us with that question. AFAIK we have nothing > documented regarding this (neither that can nor can't). > > Obviously, there are multiple ways around this problem, and at the moment > I can't tell which would be my preferred one. Yes, it's also not difficult to tweak the virtio-balloon code to avoid that issue. But if that's just an issue with ccw itself, I think it's better to tweak ccw and remain virtio-balloon unchanged. Best, Wei
