[PATCH v2] xarray: Add a BUG_ON() to ensure caller is not sibling

[Dev Jain]

Suppose xas is pointing somewhere near the end of the multi-entry batch.
Then it may happen that the computed slot already falls beyond the batch,
thus breaking the loop due to !xa_is_sibling(), and computing the wrong
order. For example, suppose we have a shift-6 node having an order-9
entry => 8 - 1 = 7 siblings, so assume the slots are at offset 0 till 7 in
this node. If xas->xa_offset is 6, then the code will compute order as
1 + xas->xa_node->shift = 7. Therefore, the order computation must start
from the beginning of the multi-slot entries, that is, the non-sibling
entry. Thus ensure that the caller is aware of this by triggering a BUG
when the entry is a sibling entry. Note that this BUG_ON() is only
active while running selftests, so there is no overhead in a running
kernel.

Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
---
v1->v2:
 - Expand changelog, add comment

Based on Torvalds' master branch.

 lib/xarray.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/lib/xarray.c b/lib/xarray.c
index 76dde3a1cacf..ae3d80f4b4ee 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1910,6 +1910,7 @@ EXPORT_SYMBOL(xa_store_range);
  * @xas: XArray operation state.
  *
  * Called after xas_load, the xas should not be in an error state.
+ * The xas should not be pointing to a sibling entry.
  *
  * Return: A number between 0 and 63 indicating the order of the entry.
  */
@@ -1920,6 +1921,8 @@ int xas_get_order(struct xa_state *xas)
 	if (!xas->xa_node)
 		return 0;
 
+	XA_NODE_BUG_ON(xas->xa_node, xa_is_sibling(xa_entry(xas->xa,
+		       xas->xa_node, xas->xa_offset)));
 	for (;;) {
 		unsigned int slot = xas->xa_offset + (1 << order);
 
-- 
2.30.2