drbd: cleanup bogus assert message

This fixes ASSERT( mdev->state.disk == D_FAILED ) in drivers/block/drbd/drbd_main.c

When we detach from local disk, we let the local refcount hit zero twice.

First, we transition to D_FAILED, so we won't give out new references
to incoming requests; we still may give out *internal* references, though.
Once the refcount hits zero [1] while in D_FAILED, we queue a transition
to D_DISKLESS to our worker.  We need to queue it, because we may be in
atomic context when putting the reference.
Once the transition to D_DISKLESS actually happened [2] from worker context,
we don't give out new internal references either.

Between hitting zero the first time [1] and actually transition to
D_DISKLESS [2], there may be a few very short lived internal get/put,
so we may hit zero more than once while being in D_FAILED, or even see a
race where a an internal get_ldev() happened while D_FAILED, but the
corresponding put_ldev() happens just after the transition to D_DISKLESS.

That's why we have the additional test_and_set_bit(GO_DISKLESS,);
and that's why the assert was placed wrong.
Since there was exactly one code path left to drbd_go_diskless(),
and that checks already for D_FAILED, drop that assert,
and fold in the drbd_queue_work().

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_int.h

@@ -1148,7 +1148,6 @@ extern int drbd_bitmap_io_from_worker(struct drbd_conf *mdev,
 		char *why, enum bm_flag flags);
 extern int drbd_bmio_set_n_write(struct drbd_conf *mdev);
 extern int drbd_bmio_clear_n_write(struct drbd_conf *mdev);
-extern void drbd_go_diskless(struct drbd_conf *mdev);
 extern void drbd_ldev_destroy(struct drbd_conf *mdev);
 
 /* Meta data layout
@@ -2053,9 +2052,11 @@ static inline void put_ldev(struct drbd_conf *mdev)
 		if (mdev->state.disk == D_DISKLESS)
 			/* even internal references gone, safe to destroy */
 			drbd_ldev_destroy(mdev);
-		if (mdev->state.disk == D_FAILED)
+		if (mdev->state.disk == D_FAILED) {
 			/* all application IO references gone. */
-			drbd_go_diskless(mdev);
+			if (!test_and_set_bit(GO_DISKLESS, &mdev->flags))
+				drbd_queue_work(&mdev->tconn->sender_work, &mdev->go_diskless);
+		}
 		wake_up(&mdev->misc_wait);
 	}
 }

drivers/block/drbd/drbd_main.c

@@ -3252,13 +3252,6 @@ static int w_go_diskless(struct drbd_work *w, int unused)
 	return 0;
 }
 
-void drbd_go_diskless(struct drbd_conf *mdev)
-{
-	D_ASSERT(mdev->state.disk == D_FAILED);
-	if (!test_and_set_bit(GO_DISKLESS, &mdev->flags))
-		drbd_queue_work(&mdev->tconn->sender_work, &mdev->go_diskless);
-}
-
 /**
  * drbd_queue_bitmap_io() - Queues an IO operation on the whole bitmap
  * @mdev:	DRBD device.