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

[Liu Zhenlong <dragonliu2018@gmail.com>]

__rtrs_get_permit() allocates a free permit from a bitmap under lockless
contention: it scans with find_first_zero_bit() and claims the bit with
test_and_set_bit_lock(), restarting the whole scan when it loses the
race.  Each retry rewinds to bit 0 and re-walks every already-set low
bit before reaching the free region again.

Under high queue depth - the RTRS/RNBD data path - the low part of
permits_map is densely set, so a lost race wastes a scan proportional
to the number of in-use permits on every retry.

Use find_next_zero_bit(), resuming from the last position, so a lost
race continues scanning from where it left off instead of from the
beginning.  When the scan reaches the end, wrap to the beginning to
exhaust the map, so a permit freed below the cursor is still found
and NULL is returned only when the map is actually full, matching the
original behavior.  The scan remains non-atomic, so the
test_and_set_bit_lock() retry is still required and the race
handling is unchanged.

A userspace model of the bitmap-allocation algorithm (not the kernel
find_*_bit primitives) quantifies the mechanism: with qdepth=512 and 14
threads holding ~98% of the bits set, a lost race in the baseline
re-scans the densely-set low region, traversing ~1.3k bit-positions per
allocation, while the patched version resumes and traverses ~260.  The
benefit is contention- and density-dependent: under low contention
(sparse map) the two are equivalent, and the wrap adds a small amount
of code over the single-scan baseline.

End-to-end RNBD/fio throughput was not measured (no RDMA hardware
available); the model isolates the allocation mechanism, not the full
IO path.

Compile-tested: arm64 defconfig + INFINIBAND_RTRS_CLIENT=m, rtrs-clt.o
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