On Sun, 11 Mar 2007, Milan Plzik wrote:
> Hello,
>
> I noticed that there could potentially be a sort of race condition in
> ohci-hcd.c file, related to interrupt handling. This caused Oops when
> removing ohci_hcd driver on one device I was working with - OHCI cell on
> SAMCOP chip used in ipaq h5000 devices. Please Cc: me as I am not
> subscribed to linux-usb-devel. Bugreport below:
>
> Summary: ohci-hcd module can possibly crash when unloading due race
> condition.
>
> Description:
> In usb_remove_hcd there is possible place where race condition can
> occur. Problematic is caused by usb_hcd_poll_rh_status, which is
> normally called periodically from timer, or from ohci_irq handler.
> Removing HCD uses roughly following algorithm:
>
> 1) delete rh_timer, so it should not be called again
> 2) call ohci_stop, which does some magic and then disables interrupts.
>
> The problem is within the "some magic" part. Its execution takes some
> time and within this time, OHCI irq can possibly occur. This results in
> call of usb_hcd_poll_rh_status, which again calls timer_mod and
> re-enables rh_timer. Consecutively, we now have ohci driver which is
> short before unloading, but has active timer. This results in paging
> fault.
>
> It is possible that the interrupt occurs because of ohci_usb_reset
> called just prior to disabling interrupts, but I have no idea whether
> this is real. This problem also might or might not be visible depending
> on debugging turned on or off.
Yes, this looks like a real bug. Does it happen reproducibly on your
device? Does the patch below fix it?
Alan Stern
Index: usb-2.6/drivers/usb/core/hcd.c
===================================================================
--- usb-2.6.orig/drivers/usb/core/hcd.c
+++ usb-2.6/drivers/usb/core/hcd.c
@@ -544,6 +544,8 @@ void usb_hcd_poll_rh_status(struct usb_h
unsigned long flags;
char buffer[4]; /* Any root hubs with > 31 ports? */
+ if (unlikely(!hcd->rh_registered))
+ return;
if (!hcd->uses_new_polling && !hcd->status_urb)
return;
@@ -1670,12 +1672,12 @@ void usb_remove_hcd(struct usb_hcd *hcd)
usb_disconnect(&hcd->self.root_hub);
mutex_unlock(&usb_bus_list_lock);
- hcd->poll_rh = 0;
- del_timer_sync(&hcd->rh_timer);
-
hcd->driver->stop(hcd);
hcd->state = HC_STATE_HALT;
+ hcd->poll_rh = 0;
+ del_timer_sync(&hcd->rh_timer);
+
if (hcd->irq >= 0)
free_irq(hcd->irq, hcd);
usb_deregister_bus(&hcd->self);
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