[PATCH v2 3/3] maple_tree: add a fast path case in mas_wr_slot_store()

[Peng Zhang]

When expanding a range in two directions, only partially overwriting the
previous and next ranges, the number of entries will not be increased, so
we can just update the pivots as a fast path. However, it may introduce
potential risks in RCU mode (although it may pass the test), because it
updates two pivots. We only enable it in non-RCU mode for now.

Signed-off-by: Peng Zhang <zhangpeng.00 at bytedance.com>
---
 lib/maple_tree.c | 36 ++++++++++++++++++++++++------------
 1 file changed, 24 insertions(+), 12 deletions(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index a96eb646e839..d3072858c280 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -4167,23 +4167,35 @@ static inline bool mas_wr_slot_store(struct ma_wr_state *wr_mas)
 {
 	struct ma_state *mas = wr_mas->mas;
 	unsigned char offset = mas->offset;
+	void __rcu **slots = wr_mas->slots;
 	bool gap = false;
 
-	if (wr_mas->offset_end - offset != 1)
-		return false;
-
-	gap |= !mt_slot_locked(mas->tree, wr_mas->slots, offset);
-	gap |= !mt_slot_locked(mas->tree, wr_mas->slots, offset + 1);
+	gap |= !mt_slot_locked(mas->tree, slots, offset);
+	gap |= !mt_slot_locked(mas->tree, slots, offset + 1);
 
-	if (mas->index == wr_mas->r_min) {
-		/* Overwriting the range and over a part of the next range. */
-		rcu_assign_pointer(wr_mas->slots[offset], wr_mas->entry);
-		wr_mas->pivots[offset] = mas->last;
-	} else {
-		/* Overwriting a part of the range and over the next range */
-		rcu_assign_pointer(wr_mas->slots[offset + 1], wr_mas->entry);
+	if (wr_mas->offset_end - offset == 1) {
+		if (mas->index == wr_mas->r_min) {
+			/* Overwriting the range and a part of the next one */
+			rcu_assign_pointer(slots[offset], wr_mas->entry);
+			wr_mas->pivots[offset] = mas->last;
+		} else {
+			/* Overwriting a part of the range and the next one */
+			rcu_assign_pointer(slots[offset + 1], wr_mas->entry);
+			wr_mas->pivots[offset] = mas->index - 1;
+			mas->offset++; /* Keep mas accurate. */
+		}
+	} else if (!mt_in_rcu(mas->tree)) {
+		/*
+		 * Expand the range, only partially overwriting the previous and
+		 * next ranges
+		 */
+		gap |= !mt_slot_locked(mas->tree, slots, offset + 2);
+		rcu_assign_pointer(slots[offset + 1], wr_mas->entry);
 		wr_mas->pivots[offset] = mas->index - 1;
+		wr_mas->pivots[offset + 1] = mas->last;
 		mas->offset++; /* Keep mas accurate. */
+	} else {
+		return false;
 	}
 
 	trace_ma_write(__func__, mas, 0, wr_mas->entry);
-- 
2.20.1