[PATCH] USB: EHCI SMP fix

This addresses an SMP-only issue with the EHCI driver, where only one CPU
should scan the schedule at a time (scanning is not re-entrant) but either
the IRQ handler or a watchdog timer could end up starting it.  Many thanks
to Olaf Hering for isolating the failure mode, and testing this fix!

Once once CPU starts scanning, any other might as well finish right
away.  This fix just adds a flag to detect that case.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/usb/host/ehci-hcd.c

@@ -695,9 +695,18 @@ static void ehci_work (struct ehci_hcd *ehci, struct pt_regs *regs)
 	timer_action_done (ehci, TIMER_IO_WATCHDOG);
 	if (ehci->reclaim_ready)
 		end_unlink_async (ehci, regs);
+
+	/* another CPU may drop ehci->lock during a schedule scan while
+	 * it reports urb completions.  this flag guards against bogus
+	 * attempts at re-entrant schedule scanning.
+	 */
+	if (ehci->scanning)
+		return;
+	ehci->scanning = 1;
 	scan_async (ehci, regs);
 	if (ehci->next_uframe != -1)
 		scan_periodic (ehci, regs);
+	ehci->scanning = 0;
 
 	/* the IO watchdog guards against hardware or driver bugs that
 	 * misplace IRQs, and should let us run completely without IRQs.

drivers/usb/host/ehci.h

@@ -53,6 +53,7 @@ struct ehci_hcd {			/* one per controller */
 	struct ehci_qh		*async;
 	struct ehci_qh		*reclaim;
 	unsigned		reclaim_ready : 1;
+	unsigned		scanning : 1;
 
 	/* periodic schedule support */
 #define	DEFAULT_I_TDPS		1024		/* some HCs can do less */