drbd: Create new current UUID as late as possible

The choice was to either delay creation of the new UUID until
IO got thawed or to delay it until the first IO request.

Both are correct, the later is more friendly to users of
dual-primary setups, that actually only write on one side.

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

1 files changed, 11 insertions, 0 deletions

diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c
index a04ec01ab3ce..461d9872d4d3 100644
--- a/drivers/block/drbd/drbd_receiver.c
+++ b/drivers/block/drbd/drbd_receiver.c
@@ -1150,6 +1150,17 @@ int drbd_submit_ee(struct drbd_conf *mdev, struct drbd_epoch_entry *e,
         unsigned n_bios = 0;
         unsigned nr_pages = (ds + PAGE_SIZE -1) >> PAGE_SHIFT;
 
+        if (atomic_read(&mdev->new_c_uuid)) {
+                if (atomic_add_unless(&mdev->new_c_uuid, -1, 1)) {
+                        drbd_uuid_new_current(mdev);
+                        drbd_md_sync(mdev);
+
+                        atomic_dec(&mdev->new_c_uuid);
+                        wake_up(&mdev->misc_wait);
+                }
+                wait_event(mdev->misc_wait, !atomic_read(&mdev->new_c_uuid));
+        }
+
         /* In most cases, we will only need one bio.  But in case the lower
          * level restrictions happen to be different at this offset on this
          * side than those of the sending peer, we may need to submit the