drbd: abort start of resync early, if it raced with connection breakage

We've seen a spurious full resync, because a connection breakage
raced with drbd_start_resync(, C_SYNC_TARGET),
and the resulting state change request intended to start the resync
ended up looking like a local invalidate.

Fix:
Double check the state inside the lock,
and don't even request that state change,
if we had connection or IO problems.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/drbd/drbd_worker.c

@@ -1653,7 +1653,9 @@ void drbd_start_resync(struct drbd_conf *mdev, enum drbd_conns side)
 	clear_bit(B_RS_H_DONE, &mdev->flags);
 
 	write_lock_irq(&global_state_lock);
-	if (!get_ldev_if_state(mdev, D_NEGOTIATING)) {
+	/* Did some connection breakage or IO error race with us? */
+	if (mdev->state.conn < C_CONNECTED
+	|| !get_ldev_if_state(mdev, D_NEGOTIATING)) {
 		write_unlock_irq(&global_state_lock);
 		mutex_unlock(mdev->state_mutex);
 		return;