Btrfs: fix potential race in extent buffer freeing

This sounds sort of impossible but it is the only thing I can think of and
at the very least it is theoretically possible so here it goes.

If we are in try_release_extent_buffer we will check that the ref count on
the extent buffer is 1 and not under IO, and then go down and clear the tree
ref.  If between this check and clearing the tree ref somebody else comes in
and grabs a ref on the eb and the marks it dirty before
try_release_extent_buffer() does it's tree ref clear we can end up with a
dirty eb that will be freed while it is still dirty which will result in a
panic.  Thanks,
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/extent_io.c

@@ -4123,11 +4123,10 @@ static void check_buffer_tree_ref(struct extent_buffer *eb)
 	 * So bump the ref count first, then set the bit.  If someone
 	 * beat us to it, drop the ref we added.
 	 */
-	if (!test_bit(EXTENT_BUFFER_TREE_REF, &eb->bflags)) {
+	spin_lock(&eb->refs_lock);
+	if (!test_and_set_bit(EXTENT_BUFFER_TREE_REF, &eb->bflags))
 		atomic_inc(&eb->refs);
-		if (test_and_set_bit(EXTENT_BUFFER_TREE_REF, &eb->bflags))
-			atomic_dec(&eb->refs);
-	}
+	spin_unlock(&eb->refs_lock);
 }
 
 static void mark_extent_buffer_accessed(struct extent_buffer *eb)
@@ -4239,9 +4238,7 @@ struct extent_buffer *alloc_extent_buffer(struct extent_io_tree *tree,
 		goto free_eb;
 	}
 	/* add one reference for the tree */
-	spin_lock(&eb->refs_lock);
 	check_buffer_tree_ref(eb);
-	spin_unlock(&eb->refs_lock);
 	spin_unlock(&tree->buffer_lock);
 	radix_tree_preload_end();