[PATCH v2] RDMA/rtrs-clt: use find_next_zero_bit() for permit allocation

[Liu Zhenlong <dragonliu2018@gmail.com>]

__rtrs_get_permit() scans permits_map with find_first_zero_bit() and
claims the bit with test_and_set_bit_lock(), restarting from bit 0 on a
lost race.  Use find_next_zero_bit() to resume from the last position so
a lost race does not rescan the already-set low bits; on reaching the
end, wrap to the beginning to exhaust the map.

Compile-tested: arm64 defconfig + INFINIBAND_RTRS_CLIENT=m, rtrs-clt.o
Reviewed-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Assisted-by: Claude:claude-opus-5
Signed-off-by: Liu Zhenlong <dragonliu2018@gmail.com>
---
 drivers/infiniband/ulp/rtrs/rtrs-clt.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/drivers/infiniband/ulp/rtrs/rtrs-clt.c b/drivers/infiniband/ulp/rtrs/rtrs-clt.c
index d34d7e5f34d6..a1df90243c41 100644
--- a/drivers/infiniband/ulp/rtrs/rtrs-clt.c
+++ b/drivers/infiniband/ulp/rtrs/rtrs-clt.c
@@ -70,19 +70,24 @@ __rtrs_get_permit(struct rtrs_clt_sess *clt, enum rtrs_clt_con_type con_type)
 {
 	size_t max_depth = clt->queue_depth;
 	struct rtrs_permit *permit;
-	int bit;
+	unsigned long bit = 0;
 
 	/*
-	 * Adapted from null_blk get_tag(). Callers from different cpus may
-	 * grab the same bit, since find_first_zero_bit is not atomic.
-	 * But then the test_and_set_bit_lock will fail for all the
-	 * callers but one, so that they will loop again.
-	 * This way an explicit spinlock is not required.
+	 * Callers from different CPUs may grab the same bit, since the bitmap
+	 * scan is not atomic. But then the test_and_set_bit_lock() will fail
+	 * for all the callers but one, so that they loop again. This way an
+	 * explicit spinlock is not required. find_next_zero_bit() resumes
+	 * from the last position so that a lost race does not rescan the
+	 * already-set low bits; if it reaches the end, wrap to the beginning
+	 * to exhaust the map and still find a permit freed below the cursor.
 	 */
 	do {
-		bit = find_first_zero_bit(clt->permits_map, max_depth);
-		if (bit >= max_depth)
-			return NULL;
+		bit = find_next_zero_bit(clt->permits_map, max_depth, bit);
+		if (bit >= max_depth) {
+			bit = find_first_zero_bit(clt->permits_map, max_depth);
+			if (bit >= max_depth)
+				return NULL;
+		}
 	} while (test_and_set_bit_lock(bit, clt->permits_map));
 
 	permit = get_permit(clt, bit);
-- 
2.55.0