btrfs: implement a nowait option for tree searches

For NOWAIT IOCBs we'll need a way to tell search to not wait on locks
or anything.  Accomplish this by adding a path->nowait flag that will
use trylocks and skip reading of metadata, returning -EAGAIN in either
of these cases.  For now we only need this for reads, so only the read
side is handled.  Add an ASSERT() to catch anybody trying to use this
for writes so they know they'll have to implement the write side.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Stefan Roesch <shr@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 25 insertions, 0 deletions

diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index 9063072b399b..0eab3cb274a1 100644
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -286,6 +286,31 @@ struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root)
 }
 
 /*
+ * Loop around taking references on and locking the root node of the tree in
+ * nowait mode until we end up with a lock on the root node or returning to
+ * avoid blocking.
+ *
+ * Return: root extent buffer with read lock held or -EAGAIN.
+ */
+struct extent_buffer *btrfs_try_read_lock_root_node(struct btrfs_root *root)
+{
+	struct extent_buffer *eb;
+
+	while (1) {
+		eb = btrfs_root_node(root);
+		if (!btrfs_try_tree_read_lock(eb)) {
+			free_extent_buffer(eb);
+			return ERR_PTR(-EAGAIN);
+		}
+		if (eb == root->node)
+			break;
+		btrfs_tree_read_unlock(eb);
+		free_extent_buffer(eb);
+	}
+	return eb;
+}
+
+/*
  * DREW locks
  * ==========
  *