diff --git a/scst/src/kunit/lib/suites/scst_kunit_lib.c b/scst/src/kunit/lib/suites/scst_kunit_lib.c
index fe33abcaf..2dbb67da1 100644
--- a/scst/src/kunit/lib/suites/scst_kunit_lib.c
+++ b/scst/src/kunit/lib/suites/scst_kunit_lib.c
@@ -7,6 +7,66 @@
 #include "scst.h"
 #endif
 
+#include "../../../scst_lib_test.h"
+
+struct scst_port_id_fixture {
+	unsigned int free_id;
+	unsigned int probes;
+	bool invalid_id;
+};
+
+static bool scst_port_id_available(u16 id, const struct scst_tgt *t)
+{
+	struct kunit *test = kunit_get_current_test();
+	struct scst_port_id_fixture *fixture = test->priv;
+
+	fixture->probes++;
+	fixture->invalid_id |= id < SCST_MIN_REL_TGT_ID;
+	return id == fixture->free_id;
+}
+
+static void scst_port_id_boundaries(struct kunit *test)
+{
+	static const struct {
+		unsigned int start;
+		unsigned int free_id;
+		unsigned int next;
+		unsigned int probes;
+		int status;
+	} cases[] = {
+		{ 1, 1, 2, 1, 0 },
+		{ 1, 3, 4, 3, 0 },
+		{ SCST_MAX_REL_TGT_ID, SCST_MAX_REL_TGT_ID, 1, 1, 0 },
+		{ SCST_MAX_REL_TGT_ID, 1, 2, 2, 0 },
+		{ 42, 0, 42, SCST_MAX_REL_TGT_ID, -EOVERFLOW },
+	};
+	struct scst_port_id_fixture fixture;
+	unsigned int i;
+
+	test->priv = &fixture;
+	kunit_activate_static_stub(test, __scst_is_relative_target_port_id_unique,
+				   scst_port_id_available);
+	for (i = 0; i < ARRAY_SIZE(cases); i++) {
+		unsigned int next = cases[i].start;
+		u16 id = 123;
+		u16 expected_id = cases[i].free_id;
+
+		if (cases[i].status)
+			expected_id = id;
+
+		memset(&fixture, 0, sizeof(fixture));
+		fixture.free_id = cases[i].free_id;
+		KUNIT_EXPECT_EQ(test, scst_alloc_relative_target_port_id(&id, &next),
+				cases[i].status);
+		KUNIT_EXPECT_EQ(test, next, cases[i].next);
+		KUNIT_EXPECT_EQ(test, id, expected_id);
+		KUNIT_EXPECT_EQ(test, fixture.probes, cases[i].probes);
+		KUNIT_EXPECT_FALSE(test, fixture.invalid_id);
+	}
+	kunit_deactivate_static_stub(test, __scst_is_relative_target_port_id_unique);
+	test->priv = NULL;
+}
+
 static void scst_lun_flat_encoding(struct kunit *test)
 {
 	/* Flat addressing: 01 in the top bits, followed by the 14-bit LUN. */
@@ -20,6 +80,7 @@ static void scst_lun_flat_encoding(struct kunit *test)
 }
 
 static struct kunit_case scst_lib_cases[] = {
+	KUNIT_CASE(scst_port_id_boundaries),
 	KUNIT_CASE(scst_lun_flat_encoding),
 	{}
 };
diff --git a/scst/src/scst_lib.c b/scst/src/scst_lib.c
index 08cdf7452..682ee69d9 100644
--- a/scst/src/scst_lib.c
+++ b/scst/src/scst_lib.c
@@ -4115,12 +4115,14 @@ restart:
 	TRACE_EXIT();
 }
 
-static bool __scst_is_relative_target_port_id_unique(uint16_t id,
-	const struct scst_tgt *t)
+SCST_KUNIT_VISIBLE
+bool __scst_is_relative_target_port_id_unique(u16 id, const struct scst_tgt *t)
 {
 	bool res = true;
 	struct scst_tgt_template *tgtt;
 
+	SCST_KUNIT_REDIRECT(__scst_is_relative_target_port_id_unique, id, t);
+
 	TRACE_ENTRY();
 
 	list_for_each_entry(tgtt, &scst_template_list,
@@ -4160,10 +4162,32 @@ bool scst_is_relative_target_port_id_unique(uint16_t id,
 	return res;
 }
 
+/* Caller serializes the cursor and target list with scst_mutex. */
+SCST_KUNIT_VISIBLE
+int scst_alloc_relative_target_port_id(u16 *id, unsigned int *next)
+{
+	unsigned int candidate = *next;
+
+	do {
+		bool available = __scst_is_relative_target_port_id_unique(candidate, NULL);
+		unsigned int selected = candidate;
+
+		if (++candidate > SCST_MAX_REL_TGT_ID)
+			candidate = SCST_MIN_REL_TGT_ID;
+		if (available) {
+			*id = selected;
+			*next = candidate;
+			return 0;
+		}
+	} while (candidate != *next);
+
+	return -EOVERFLOW;
+}
+
 int gen_relative_target_port_id(uint16_t *id)
 {
-	int res = -EOVERFLOW;
-	static unsigned long rti = SCST_MIN_REL_TGT_ID, rti_prev;
+	static unsigned int next = SCST_MIN_REL_TGT_ID;
+	int res;
 
 	TRACE_ENTRY();
 
@@ -4171,21 +4195,10 @@ int gen_relative_target_port_id(uint16_t *id)
 	if (res != 0)
 		goto out;
 
-	rti_prev = rti;
-	do {
-		if (__scst_is_relative_target_port_id_unique(rti, NULL)) {
-			*id = (uint16_t)rti++;
-			res = 0;
-			goto out_unlock;
-		}
-		rti++;
-		if (rti > SCST_MAX_REL_TGT_ID)
-			rti = SCST_MIN_REL_TGT_ID;
-	} while (rti != rti_prev);
+	res = scst_alloc_relative_target_port_id(id, &next);
+	if (res)
+		PRINT_ERROR("%s", "Unable to create unique relative target port id");
 
-	PRINT_ERROR("%s", "Unable to create unique relative target port id");
-
-out_unlock:
 	mutex_unlock(&scst_mutex);
 
 out:
diff --git a/scst/src/scst_lib_test.h b/scst/src/scst_lib_test.h
index 8769db781..dd82a382c 100644
--- a/scst/src/scst_lib_test.h
+++ b/scst/src/scst_lib_test.h
@@ -8,6 +8,8 @@
 
 struct scst_write_same_priv;
 
+bool __scst_is_relative_target_port_id_unique(u16 id, const struct scst_tgt *t);
+int scst_alloc_relative_target_port_id(u16 *id, unsigned int *next);
 int scst_ws_build_write_cdb(const struct scst_write_same_priv *wsp,
 			    u8 *write_cdb, s64 lba, int blocks);
 void scst_del_free_acg_dev(struct scst_acg_dev *acg_dev, bool del_sysfs);