On 11/08/2018 04:43 PM, Greg Kroah-Hartman wrote: > > [EXTERNAL EMAIL] > Please report any suspicious attachments, links, or requests for sensitive information. > > > On Thu, Nov 08, 2018 at 03:32:58PM -0700, Keith Busch 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 >>>> instead of systematic, in the sense that I don't know how we convince >>>> ourselves that this (and only this) is the correct place to put it. >>> >>> 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. >> >> AIUI, we're not trying to create code to rely on this. This more about >> reducing reliance on hardware. If the software misses the race once and >> accesses disconnected device memory, that's usually not a big deal to >> let hardware sort it out, but the point is not to push our luck. > > Then why even care about this call at all? If you need to really know > if the read worked, you have to check the value. If the value is FF > then you have a huge hint that the hardware is now gone. And you can > rely on it being gone, you can never rely on making the call to the > function to check if the hardware is there to be still valid any point > in time after the call returns. In the case that we're trying to fix, this code executing is a result of the device being gone, so we can guarantee race-free operation. I agree that there is a race, in the general case. As far as checking the result for all F's, that's not an option when firmware crashes the system as a result of the mmio read/write. It's never pretty when firmware gets involved. Alex
