usb: cdc-acm: use irqsave() in USB's complete callback

The USB completion callback does not disable interrupts while acquiring
the lock. We want to remove the local_irq_disable() invocation from
__usb_hcd_giveback_urb() and therefore it is required for the callback
handler to disable the interrupts while acquiring the lock.
The callback may be invoked either in IRQ or BH context depending on the
USB host controller.
Use the _irqsave() variant of the locking primitives.

Cc: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

@@ -276,6 +276,7 @@ static void acm_process_notification(struct acm *acm, unsigned char *buf)
 {
 	int newctrl;
 	int difference;
+	unsigned long flags;
 	struct usb_cdc_notification *dr = (struct usb_cdc_notification *)buf;
 	unsigned char *data = buf + sizeof(struct usb_cdc_notification);
 
@@ -303,7 +304,7 @@ static void acm_process_notification(struct acm *acm, unsigned char *buf)
 		}
 
 		difference = acm->ctrlin ^ newctrl;
-		spin_lock(&acm->read_lock);
+		spin_lock_irqsave(&acm->read_lock, flags);
 		acm->ctrlin = newctrl;
 		acm->oldcount = acm->iocount;
 
@@ -321,7 +322,7 @@ static void acm_process_notification(struct acm *acm, unsigned char *buf)
 			acm->iocount.parity++;
 		if (difference & ACM_CTRL_OVERRUN)
 			acm->iocount.overrun++;
-		spin_unlock(&acm->read_lock);
+		spin_unlock_irqrestore(&acm->read_lock, flags);
 
 		if (difference)
 			wake_up_all(&acm->wioctl);