drbd: Prepare epochs per connection

An epoch object needs a pointer to the mdev it was received for.
This is necessary to be able to send the barrier ack packet for
the same volume as the original barrier packet was assigned to.

This prepares the next step, in which the (receiver side)
epoch list is moved from the device (mdev) to the connection (tconn)
object.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_int.h

@@ -585,6 +585,7 @@ struct drbd_tl_epoch {
 };
 
 struct drbd_epoch {
+	struct drbd_conf *mdev;
 	struct list_head list;
 	unsigned int barrier_nr;
 	atomic_t epoch_size; /* increased on every request added. */

drivers/block/drbd/drbd_receiver.c

@@ -1152,11 +1152,11 @@ static enum finish_epoch drbd_may_finish_epoch(struct drbd_conf *mdev,
 		    (test_bit(DE_HAVE_BARRIER_NUMBER, &epoch->flags) || ev & EV_CLEANUP)) {
 			if (!(ev & EV_CLEANUP)) {
 				spin_unlock(&mdev->epoch_lock);
-				drbd_send_b_ack(mdev, epoch->barrier_nr, epoch_size);
+				drbd_send_b_ack(epoch->mdev, epoch->barrier_nr, epoch_size);
 				spin_lock(&mdev->epoch_lock);
 			}
 			if (test_bit(DE_HAVE_BARRIER_NUMBER, &epoch->flags))
-				dec_unacked(mdev);
+				dec_unacked(epoch->mdev);
 
 			if (mdev->current_epoch != epoch) {
 				next_epoch = list_entry(epoch->list.next, struct drbd_epoch, list);
@@ -1349,6 +1349,7 @@ static int receive_Barrier(struct drbd_tconn *tconn, struct packet_info *pi)
 	inc_unacked(mdev);
 
 	mdev->current_epoch->barrier_nr = p->barrier;
+	mdev->current_epoch->mdev = mdev;
 	rv = drbd_may_finish_epoch(mdev, mdev->current_epoch, EV_GOT_BARRIER_NR);
 
 	/* P_BARRIER_ACK may imply that the corresponding extent is dropped from