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SubjectRe: [PATCH] ohci-hcd: Fix race condition caused by ohci_urb_enqueue() and io_watchdog_func()
On Thu, 1 Feb 2018, Yoshida, Shigeru wrote:

> Hi Alan,
>
> Thank you for your commenting.
>
> On Wed, 31 Jan 2018 11:02:47 -0500, Alan Stern wrote:
> >> 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.
>
> It's reasonable since ohci->prev_frame_no is not used while the
> watchdog timer is stopped.
>
> I think we must choose an invalid frame number for the special
> sentinel value, but I'm not sure which value is adequate for it.
> Is 0xffffff00 an invalid frame number, otherwise how about simply
> -1(0xffffffff)?

Well, the frame_no register is 32 bits wide, but only the 16 low-order
bits are meaningful. ohci_frame_no() strips off the high-order 16
bits, so any value with one of those bits set would be acceptable.
(Besides, valid frame numbers only go up to 2047.)

I chose 0xffffff00 because PCI reads from a non-working device
generally get a value with all the bits set. But since the upper 16
bits are masked away anyhow, it doesn't matter. -1u would be fine.

Alan Stern

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