tty/amiserial: avoid interruptible_sleep_on

interruptible_sleep_on is generally problematic and we want to get
rid of it. In case of TIOCMIWAIT, that race is actually in user
space and does not get fixed since we can only detect changes after
entering the ioctl handler, but it removes one more caller.

This instance can not be trivially replaced with wait_event, so
I chose to open-code the wait loop using prepare_to_wait/finish_wait.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/amiserial.c

@@ -1248,6 +1248,8 @@ static int rs_ioctl(struct tty_struct *tty,
 	struct async_icount cprev, cnow;	/* kernel counter temps */
 	void __user *argp = (void __user *)arg;
 	unsigned long flags;
+	DEFINE_WAIT(wait);
+	int ret;
 
 	if (serial_paranoia_check(info, tty->name, "rs_ioctl"))
 		return -ENODEV;
@@ -1288,25 +1290,33 @@ static int rs_ioctl(struct tty_struct *tty,
 			cprev = info->icount;
 			local_irq_restore(flags);
 			while (1) {
-				interruptible_sleep_on(&info->tport.delta_msr_wait);
-				/* see if a signal did it */
-				if (signal_pending(current))
-					return -ERESTARTSYS;
+				prepare_to_wait(&info->tport.delta_msr_wait,
+						&wait, TASK_INTERRUPTIBLE);
 				local_irq_save(flags);
 				cnow = info->icount; /* atomic copy */
 				local_irq_restore(flags);
 				if (cnow.rng == cprev.rng && cnow.dsr == cprev.dsr && 
-				    cnow.dcd == cprev.dcd && cnow.cts == cprev.cts)
-					return -EIO; /* no change => error */
+				    cnow.dcd == cprev.dcd && cnow.cts == cprev.cts) {
+					ret = -EIO; /* no change => error */
+					break;
+				}
 				if ( ((arg & TIOCM_RNG) && (cnow.rng != cprev.rng)) ||
 				     ((arg & TIOCM_DSR) && (cnow.dsr != cprev.dsr)) ||
 				     ((arg & TIOCM_CD)  && (cnow.dcd != cprev.dcd)) ||
 				     ((arg & TIOCM_CTS) && (cnow.cts != cprev.cts)) ) {
-					return 0;
+					ret = 0;
+					break;
+				}
+				schedule();
+				/* see if a signal did it */
+				if (signal_pending(current)) {
+					ret = -ERESTARTSYS;
+					break;
 				}
 				cprev = cnow;
 			}
-			/* NOTREACHED */
+			finish_wait(&info->tport.delta_msr_wait, &wait);
+			return ret;
 
 		case TIOCSERGWILD:
 		case TIOCSERSWILD: