On Fri, Nov 09, 2018 at 08:29:53AM +0100, Lukas Wunner wrote: > On Thu, Nov 08, 2018 at 02:01:17PM -0800, Greg Kroah-Hartman wrote: > > On Thu, Nov 08, 2018 at 02:09:17PM -0600, Bjorn Helgaas wrote: > > > I'm having second thoughts about this. One thing I'm uncomfortable > > > with is that sprinkling pci_dev_is_disconnected() around feels ad hoc > > > > I think my stance always has been that this call is not good at all > > because once you call it you never really know if it is still true as > > the device could have been removed right afterward. > > > > So almost any code that relies on it is broken, there is no locking and > > it can and will race and you will loose. > > Hm, to be honest if that's your impression I think you must have missed a > large portion of the discussion we've been having over the past 2 years. > > Please consider reading this LWN article, particularly the "Surprise > removal" section, to get up to speed: > > https://lwn.net/Articles/767885/ > > You seem to be assuming that all we care about is the *return value* of > an mmio read. However a transaction to a surprise removed device has > side effects beyond returning all ones, such as a Completion Timeout > which, with thousands of transactions in flight, added up to many seconds > to handle removal of an NVMe array and occasionally caused MCEs. Again, I still claim this is broken hardware/firmware :) > It is not an option to just blindly carry out device accesses even though > it is known the device is gone, Completion Timeouts be damned. I don't disagree with you at all, and your other email is great with summarizing the issues here. What I do object to is somehow relying on that function call as knowing that the device really is present or not. It's a good hint, yes, but driver authors still have to be able to handle the bad data coming back from when the call races with the device being removed. > However there is more to it than just Completion Timeouts, this is all > detailed in the LWN article. And that's a great article and your work here is much appreciated. I think we are in violent agreement :) thanks, greg k-h
