btrfs: remove useless logic when finding parent nodes

At find_parent_nodes(), at its last step, when iterating over all direct
references, we are checking if we have a share context and if we have
a reference with a different root from the one in the share context.
However that logic is pointless because of two reasons:

1) After the previous patch in the series (subject "btrfs: remove roots
   ulist when checking data extent sharedness"), the roots argument is
   always NULL when using a share check context (struct share_check), so
   this code is never triggered;

2) Even before that previous patch, we could not hit this code because
   if we had a reference with a root different from the one in our share
   context, then we would have exited earlier when doing either of the
   following:

      - Adding a second direct ref to the direct refs red black tree
        resulted in extent_is_shared() returning true when called from
        add_direct_ref() -> add_prelim_ref(), after processing delayed
        references or while processing references in the extent tree;

      - When adding a second reference to the indirect refs red black
        tree (same as above, extent_is_shared() returns true);

      - If we only have one indirect reference and no direct references,
        then when resolving it at resolve_indirect_refs() we immediately
        return that the target extent is shared, therefore never reaching
        that loop that iterates over all direct references at
        find_parent_nodes();

      - If we have 1 indirect reference and 1 direct reference, then we
        also exit early because extent_is_shared() ends up returning true
        when called through add_prelim_ref() (by add_direct_ref() or
        add_indirect_ref()) or add_delayed_refs(). Same applies as when
        having a combination of direct, indirect and indirect with missing
        key references.

   This logic had been obsoleted since commit 3ec4d323 ("btrfs:
   allow backref search checks for shared extents"), which introduced the
   early exits in case an extent is shared.

So just remove that logic, and assert at find_parent_nodes() that when we
have a share context we don't have a roots ulist and that we haven't found
the extent to be directly shared after processing delayed references and
all references from the extent tree.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/backref.c

@@ -1219,6 +1219,10 @@ static int find_parent_nodes(struct btrfs_trans_handle *trans,
 		.indirect_missing_keys = PREFTREE_INIT
 	};
 
+	/* Roots ulist is not needed when using a sharedness check context. */
+	if (sc)
+		ASSERT(roots == NULL);
+
 	key.objectid = bytenr;
 	key.offset = (u64)-1;
 	if (btrfs_fs_incompat(fs_info, SKINNY_METADATA))
@@ -1310,6 +1314,20 @@ static int find_parent_nodes(struct btrfs_trans_handle *trans,
 		}
 	}
 
+	/*
+	 * If we have a share context and we reached here, it means the extent
+	 * is not directly shared (no multiple reference items for it),
+	 * otherwise we would have exited earlier with a return value of
+	 * BACKREF_FOUND_SHARED after processing delayed references or while
+	 * processing inline or keyed references from the extent tree.
+	 * The extent may however be indirectly shared through shared subtrees
+	 * as a result from creating snapshots, so we determine below what is
+	 * its parent node, in case we are dealing with a metadata extent, or
+	 * what's the leaf (or leaves), from a fs tree, that has a file extent
+	 * item pointing to it in case we are dealing with a data extent.
+	 */
+	ASSERT(extent_is_shared(sc) == 0);
+
 	btrfs_release_path(path);
 
 	ret = add_missing_keys(fs_info, &preftrees, path->skip_locking == 0);
@@ -1347,11 +1365,6 @@ static int find_parent_nodes(struct btrfs_trans_handle *trans,
 		 * and would retain their original ref->count < 0.
 		 */
 		if (roots && ref->count && ref->root_id && ref->parent == 0) {
-			if (sc && ref->root_id != sc->root_objectid) {
-				ret = BACKREF_FOUND_SHARED;
-				goto out;
-			}
-
 			/* no parent == root of tree */
 			ret = ulist_add(roots, ref->root_id, 0, GFP_NOFS);
 			if (ret < 0)