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

[Haiqing Bai]

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 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.

Author: Yoshida, Shigeru <Shigeru.Yoshida@xxxxxxxxxxxxx>
Signed-off-by: Haiqing Bai <Haiqing.Bai@xxxxxxxxxxxxx>
---
 drivers/usb/host/ohci-hcd.c | 7 +++++++
 drivers/usb/host/ohci-hub.c | 4 +++-
 drivers/usb/host/ohci.h     | 1 +
 3 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/host/ohci-hcd.c b/drivers/usb/host/ohci-hcd.c
index ee9676349333..2c7fa0c05854 100644
--- a/drivers/usb/host/ohci-hcd.c
+++ b/drivers/usb/host/ohci-hcd.c
@@ -233,10 +233,12 @@ static int ohci_urb_enqueue (
 		/* Start up the I/O watchdog timer, if it's not running */
 		if (!timer_pending(&ohci->io_watchdog) &&
 				list_empty(&ohci->eds_in_use) &&
+				!ohci->timer_running &&
 				!(ohci->flags & OHCI_QUIRK_QEMU)) {
 			ohci->prev_frame_no = ohci_frame_no(ohci);
 			mod_timer(&ohci->io_watchdog,
 					jiffies + IO_WATCHDOG_DELAY);
+			ohci->timer_running = 1;
 		}
 		list_add(&ed->in_use_list, &ohci->eds_in_use);
 
@@ -501,6 +503,7 @@ static int ohci_init (struct ohci_hcd *ohci)
 		return 0;
 
 	timer_setup(&ohci->io_watchdog, io_watchdog_func, 0);
+	ohci->timer_running = 0;
 
 	ohci->hcca = dma_alloc_coherent (hcd->self.controller,
 			sizeof(*ohci->hcca), &ohci->hcca_dma, GFP_KERNEL);
@@ -732,6 +735,7 @@ static void io_watchdog_func(struct timer_list *t)
 	struct td	*td, *td_start, *td_next;
 	unsigned	frame_no;
 	unsigned long	flags;
+	int		timer_running = 0;
 
 	spin_lock_irqsave(&ohci->lock, flags);
 
@@ -841,10 +845,12 @@ static void io_watchdog_func(struct timer_list *t)
 					&ohci->regs->donehead);
 			mod_timer(&ohci->io_watchdog,
 					jiffies + IO_WATCHDOG_DELAY);
+			timer_running = 1;
 		}
 	}
 
  done:
+	ohci->timer_running = timer_running;
 	spin_unlock_irqrestore(&ohci->lock, flags);
 }
 
@@ -973,6 +979,7 @@ static void ohci_stop (struct usb_hcd *hcd)
 	if (quirk_nec(ohci))
 		flush_work(&ohci->nec_work);
 	del_timer_sync(&ohci->io_watchdog);
+	ohci->timer_running = 0;
 
 	ohci_writel (ohci, OHCI_INTR_MIE, &ohci->regs->intrdisable);
 	ohci_usb_reset(ohci);
diff --git a/drivers/usb/host/ohci-hub.c b/drivers/usb/host/ohci-hub.c
index fb7aaa3b9d06..2aa21ee39cc2 100644
--- a/drivers/usb/host/ohci-hub.c
+++ b/drivers/usb/host/ohci-hub.c
@@ -311,8 +311,10 @@ static int ohci_bus_suspend (struct usb_hcd *hcd)
 		rc = ohci_rh_suspend (ohci, 0);
 	spin_unlock_irq (&ohci->lock);
 
-	if (rc == 0)
+	if (rc == 0) {
 		del_timer_sync(&ohci->io_watchdog);
+		ohci->timer_running = 0;
+	}
 	return rc;
 }
 
diff --git a/drivers/usb/host/ohci.h b/drivers/usb/host/ohci.h
index 508a803139dd..84f96558d667 100644
--- a/drivers/usb/host/ohci.h
+++ b/drivers/usb/host/ohci.h
@@ -427,6 +427,7 @@ struct ohci_hcd {
 	unsigned		wdh_cnt, prev_wdh_cnt;
 	u32			prev_donehead;
 	struct timer_list	io_watchdog;
+	int			timer_running;
 
 	struct work_struct	nec_work;	/* Worker for NEC quirk */
 
-- 
2.11.0