On Mon, 6 Jan 2020, Kai-Heng Feng wrote: > > On Jan 5, 2020, at 00:20, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > > > On Sat, 4 Jan 2020, Kai-Heng Feng wrote: > > > >>>>>> @@ -3533,9 +3533,17 @@ int usb_port_resume(struct usb_device *udev, pm_message_t msg) > >>>>>> } > >>>>>> > >>>>>> /* see 7.1.7.7; affects power usage, but not budgeting */ > >>>>>> - if (hub_is_superspeed(hub->hdev)) > >>>>>> + if (hub_is_superspeed(hub->hdev)) { > >>>>>> + if (hub->hdev->quirks & USB_QUIRK_DISABLE_LPM_ON_U0) { > >>>>>> + usb_lock_device(hub->hdev); > >>>>>> + usb_unlocked_disable_lpm(hub->hdev); > >>>>>> + } > >>>>>> status = hub_set_port_link_state(hub, port1, USB_SS_PORT_LS_U0); > >>>>>> - else > >>>>>> + if (hub->hdev->quirks & USB_QUIRK_DISABLE_LPM_ON_U0) { > >>>>>> + usb_unlocked_enable_lpm(hub->hdev); > >>>>>> + usb_unlock_device(hub->hdev); > >>>>> > >>>>> The locking here seems questionable. Doesn't this code sometimes get > >>>>> called with the hub already locked? Or with the child device locked > >>>>> (in which case locking the hub would violate the normal locking order: > >>>>> parent first, child second)? > >>> > >>> I did a little checking. In many cases the child device _will_ be > >>> locked at this point. > >>> > >>>> Maybe introduce a new lock? The lock however will only be used by this specific hub. > >>>> But I still want the LPM can be enabled for this hub. > >>> > >>> Do you really need to lock the hub at all? What would the lock protect > >>> against? > >> > >> There can be multiple usb_port_resume() run at the same time for different ports, so this is to prevent LPM enable/disable race. > > > > But there can't really be an LPM enable/disable race, can there? The > > individual function calls are protected by the bandwidth mutex taken by > > the usb_unlocked_{en|dis}able_lpm routines, and the overall LPM setting > > is controlled by the hub device's lpm_disable_counter. > > For enable/disable LPM itself, there's no race. > But the lock here is to protect hub_set_port_link_state(). > If we don't lock the hub, other instances of usb_port_resume() > routine can enable LPM and we want the LPM stays disabled until > hub_set_port_link_state() is done. That's what I was trying to explain above. Other instances of usb_port_resume() _can't_ enable LPM while this instance is running, because the lpm_disable_counter value will be > 0. Alan Stern