On Tue, 18 Nov 2014, Julius Werner wrote: > >> You should be aware that it's not safe to use hcd->state for anything > >> in a host controller driver. That field is owned by usbcore, not by > >> the HCD, and it is not protected by any locks. > >> > >> Thus, for example, hcd->state does not get set to HC_STATE_SUSPENDED > >> until some time after the bus_suspend routine has returned. A > >> port-change interrupt might occur during that time interval. > > Looks like there is explicit code in hcd_bus_suspend() to check for > that race condition right after setting hcd->state, or do I > misinterpret that (the "Did we race with a root-hub wakeup event?" > part)? That code doesn't quite do what you think. For example: CPU 1 CPU 2 ----- ----- hcd_bus_suspend(): call hcd->bus_suspend(): root hub gets suspended Wakeup IRQ arrives and is ignored because hcd->state is still HC_STATE_QUIESCING set hcd->state to HC_STATE_SUSPENDED Did we race with a wakeup event? No because usb_hcd_resume_root_hub() wasn't called. Result: the wakeup event is lost. > Also, it seems xhci_bus_suspend() explicitly sets 'hcd->state = > HC_STATE_SUSPENDED' before giving up the spinlock for some > undocumented reason, maybe to avoid exactly this problem. We could > just copy that trick if the hcd.c solution isn't enough (although the > DWC2 bus_suspend/bus_resume in the other patch don't grab that > spinlock right now, where I'm also not so sure if that's a good > idea...). It's better for HCDs to avoid testing hcd->state at all. They should set it to appropriate values at the right times, because usbcore checks it, but they should not test it. This is why ehci-hcd, ohci-hcd, and uhci-hcd all have a private rh_state variable, and they use it while holding their respective private spinlocks. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
