On Wed, 31 Jan 2018, Haiqing Bai wrote: > Running io_watchdog_func() while ohci_urb_enqueue() is running can > cause a race condition where ohci->prev_frame_no is corrupted and the > watchdog can mis-detect following error: > > ohci-platform 664a0800.usb: frame counter not updating; disabled > ohci-platform 664a0800.usb: HC died; cleaning up > > Specifically, following scenario causes a race condition: > > 1. ohci_urb_enqueue() calls spin_lock_irqsave(&ohci->lock, flags) > and enters the critical section > 2. ohci_urb_enqueue() calls timer_pending(&ohci->io_watchdog) and it > returns false > 3. ohci_urb_enqueue() sets ohci->prev_frame_no to a frame number > read by ohci_frame_no(ohci) > 4. ohci_urb_enqueue() schedules io_watchdog_func() with mod_timer() > 5. ohci_urb_enqueue() calls spin_unlock_irqrestore(&ohci->lock, > flags) and exits the critical section > 6. Later, ohci_urb_enqueue() is called > 7. ohci_urb_enqueue() calls spin_lock_irqsave(&ohci->lock, flags) > and enters the critical section > 8. The timer scheduled on step 4 expires and io_watchdog_func() runs > 9. io_watchdog_func() calls spin_lock_irqsave(&ohci->lock, flags) > and waits on it because ohci_urb_enqueue() is already in the > critical section on step 7 > 10. ohci_urb_enqueue() calls timer_pending(&ohci->io_watchdog) and it > returns false > 11. ohci_urb_enqueue() sets ohci->prev_frame_no to new frame number > read by ohci_frame_no(ohci) because the frame number proceeded > between step 3 and 6 > 12. ohci_urb_enqueue() schedules io_watchdog_func() with mod_timer() > 13. ohci_urb_enqueue() calls spin_unlock_irqrestore(&ohci->lock, > flags) and exits the critical section, then wake up > io_watchdog_func() which is waiting on step 9 > 14. io_watchdog_func() enters the critical section > 15. io_watchdog_func() calls ohci_frame_no(ohci) and set frame_no > variable to the frame number > 16. io_watchdog_func() compares frame_no and ohci->prev_frame_no > > On step 16, because this calling of io_watchdog_func() is scheduled on > step 4, the frame number set in ohci->prev_frame_no is expected to the > number set on step 3. However, ohci->prev_frame_no is overwritten on > step 11. Because step 16 is executed soon after step 11, the frame > number might not proceed, so ohci->prev_frame_no must equals to > frame_no. That is a nasty bug! > To address above scenario, this patch introduces timer_running flag to > ohci_hcd structure. Setting true to ohci->timer_running indicates > io_watchdog_func() is scheduled or is running. ohci_urb_enqueue() > checks the flag when it schedules the watchdog (step 4 and 12 above), > so ohci->prev_frame_no is not overwritten while io_watchdog_func() is > running. Instead of adding an extra flag variable, which has to be kept in sync with the timer routine, how about defining a special sentinel value for prev_frame_no? For example: #define IO_WATCHDOG_OFF 0xffffff00 Then whenever the timer isn't scheduled or running, set ohci->prev_frame_no to IO_WATCHDOG_OFF. And instead of testing timer_pending(), compare prev_frame_no to this special value. I think that approach will be slightly more robust. 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
