drbd: Reduce the time an empty resync takes usually

This mitigates changes introduced with commit:
http://git.drbd.org/?p=drbd-8.3.git;a=commit;h=4b6803a3276652da3737Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_int.h

@@ -800,6 +800,7 @@ enum {
 	RESIZE_PENDING,		/* Size change detected locally, waiting for the response from
 				 * the peer, if it changed there as well. */
 	CONN_DRY_RUN,		/* Expect disconnect after resync handshake. */
+	GOT_PING_ACK,		/* set when we receive a ping_ack packet, misc wait gets woken */
 };
 
 struct drbd_bitmap; /* opaque for drbd_conf */

drivers/block/drbd/drbd_receiver.c

@@ -4074,6 +4074,8 @@ static int got_PingAck(struct drbd_conf *mdev, struct p_header *h)
 {
 	/* restore idle timeout */
 	mdev->meta.socket->sk->sk_rcvtimeo = mdev->net_conf->ping_int*HZ;
+	if (!test_and_set_bit(GOT_PING_ACK, &mdev->flags))
+		wake_up(&mdev->misc_wait);
 
 	return TRUE;
 }

drivers/block/drbd/drbd_worker.c

@@ -1289,6 +1289,14 @@ int drbd_alter_sa(struct drbd_conf *mdev, int na)
 	return retcode;
 }
 
+static void ping_peer(struct drbd_conf *mdev)
+{
+	clear_bit(GOT_PING_ACK, &mdev->flags);
+	request_ping(mdev);
+	wait_event(mdev->misc_wait,
+		   test_bit(GOT_PING_ACK, &mdev->flags) || mdev->state.conn < C_CONNECTED);
+}
+
 /**
  * drbd_start_resync() - Start the resync process
  * @mdev:	DRBD device.
@@ -1383,9 +1391,7 @@ void drbd_start_resync(struct drbd_conf *mdev, enum drbd_conns side)
 
 		if (mdev->rs_total == 0) {
 			/* Peer still reachable? Beware of failing before-resync-target handlers! */
-			request_ping(mdev);
-			__set_current_state(TASK_INTERRUPTIBLE);
-			schedule_timeout(mdev->net_conf->ping_timeo*HZ/9); /* 9 instead 10 */
+			ping_peer(mdev);
 			drbd_resync_finished(mdev);
 			return;
 		}