tty: hvc: hvc_write() may sleep

Rework the hvc_write loop to drop and re-take the spinlock on each
iteration, add a cond_resched. Don't bother with an initial hvc_push
initially, which makes the logic simpler -- just do a hvc_push on
each time around the loop.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

drivers/tty/hvc/hvc_console.c

@@ -493,13 +493,12 @@ static int hvc_write(struct tty_struct *tty, const unsigned char *buf, int count
 	if (hp->port.count <= 0)
 		return -EIO;
 
+	while (count > 0) {
 		spin_lock_irqsave(&hp->lock, flags);
 
-	/* Push pending writes */
-	if (hp->n_outbuf > 0)
-		hvc_push(hp);
+		rsize = hp->outbuf_size - hp->n_outbuf;
 
-	while (count > 0 && (rsize = hp->outbuf_size - hp->n_outbuf) > 0) {
+		if (rsize) {
 			if (rsize > count)
 				rsize = count;
 			memcpy(hp->outbuf + hp->n_outbuf, buf, rsize);
@@ -507,10 +506,17 @@ static int hvc_write(struct tty_struct *tty, const unsigned char *buf, int count
 			buf += rsize;
 			hp->n_outbuf += rsize;
 			written += rsize;
-		hvc_push(hp);
 		}
+
+		if (hp->n_outbuf > 0)
+			hvc_push(hp);
+
 		spin_unlock_irqrestore(&hp->lock, flags);
 
+		if (count)
+			cond_resched();
+	}
+
 	/*
 	 * Racy, but harmless, kick thread if there is still pending data.
 	 */