ohci-hcd: Fix race condition caused by ohci_urb_enqueue() and io_watchdog_func()

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.

To address above scenario, this patch introduces a special sentinel
value IO_WATCHDOG_OFF and set this value to ohci->prev_frame_no when
the watchdog is not pending or running. When ohci_urb_enqueue()
schedules the watchdog (step 4 and 12 above), it compares
ohci->prev_frame_no to IO_WATCHDOG_OFF so that ohci->prev_frame_no is
not overwritten while io_watchdog_func() is running.

Signed-off-by: Shigeru Yoshida <[e-mail]>
Signed-off-by: Haiqing Bai <[e-mail]>
Acked-by: Alan Stern <[e-mail]>
Cc: stable <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/host/ohci-hcd.c

···
 
 #define	STATECHANGE_DELAY	msecs_to_jiffies(300)
 #define	IO_WATCHDOG_DELAY	msecs_to_jiffies(275)
+#define	IO_WATCHDOG_OFF		0xffffff00
 
 #include "ohci.h"
 #include "pci-quirks.h"
···
 		}
 
 		/* Start up the I/O watchdog timer, if it's not running */
-		if (!timer_pending(&ohci->io_watchdog) &&
+		if (ohci->prev_frame_no == IO_WATCHDOG_OFF &&
 				list_empty(&ohci->eds_in_use) &&
 				!(ohci->flags & OHCI_QUIRK_QEMU)) {
 			ohci->prev_frame_no = ohci_frame_no(ohci);
···
 		return 0;
 
 	timer_setup(&ohci->io_watchdog, io_watchdog_func, 0);
+	ohci->prev_frame_no = IO_WATCHDOG_OFF;
 
 	ohci->hcca = dma_alloc_coherent (hcd->self.controller,
 			sizeof(*ohci->hcca), &ohci->hcca_dma, GFP_KERNEL);
···
 	u32		head;
 	struct ed	*ed;
 	struct td	*td, *td_start, *td_next;
-	unsigned	frame_no;
+	unsigned	frame_no, prev_frame_no = IO_WATCHDOG_OFF;
 	unsigned long	flags;
 
 	spin_lock_irqsave(&ohci->lock, flags);
···
 			}
 		}
 		if (!list_empty(&ohci->eds_in_use)) {
-			ohci->prev_frame_no = frame_no;
+			prev_frame_no = frame_no;
 			ohci->prev_wdh_cnt = ohci->wdh_cnt;
 			ohci->prev_donehead = ohci_readl(ohci,
 					&ohci->regs->donehead);
···
 	}
 
  done:
+	ohci->prev_frame_no = prev_frame_no;
 	spin_unlock_irqrestore(&ohci->lock, flags);
 }
 
···
 	if (quirk_nec(ohci))
 		flush_work(&ohci->nec_work);
 	del_timer_sync(&ohci->io_watchdog);
+	ohci->prev_frame_no = IO_WATCHDOG_OFF;
 
 	ohci_writel (ohci, OHCI_INTR_MIE, &ohci->regs->intrdisable);
 	ohci_usb_reset(ohci);

drivers/usb/host/ohci-hub.c

···
 		rc = ohci_rh_suspend (ohci, 0);
 	spin_unlock_irq (&ohci->lock);
 
-	if (rc == 0)
+	if (rc == 0) {
 		del_timer_sync(&ohci->io_watchdog);
+		ohci->prev_frame_no = IO_WATCHDOG_OFF;
+	}
 	return rc;
 }