tests: lib: lockfree: Fix MPSC test consumer node return logic

tests/lib/lockfree/boards/qemu_cortex_a53_qemu_cortex_a53_smp.conf

@@ -0,0 +1,2 @@
+# Configuration overlay for 4-CPU testing on QEMU Cortex-A53
+CONFIG_MP_MAX_NUM_CPUS=4

tests/lib/lockfree/boards/qemu_cortex_a53_qemu_cortex_a53_smp.overlay

@@ -0,0 +1,22 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Device tree overlay to add 2 more CPUs to QEMU Cortex-A53 SMP
+ * for testing 4-CPU configurations
+ */
+
+/ {
+	cpus {
+		cpu@2 {
+			device_type = "cpu";
+			compatible = "arm,cortex-a53";
+			reg = <2>;
+		};
+
+		cpu@3 {
+			device_type = "cpu";
+			compatible = "arm,cortex-a53";
+			reg = <3>;
+		};
+	};
+};

tests/lib/lockfree/src/test_mpsc.c

@@ -127,7 +127,10 @@ static void mpsc_consumer(void *p1, void *p2, void *p3)
 
 		nn = CONTAINER_OF(n, struct test_mpsc_node, n);
 
-		spsc_acquire(node_q[nn->id]);
+		/* Return node to producer's free queue - must retry if queue is full */
+		while (spsc_acquire(node_q[nn->id]) == NULL) {
+			k_yield();
+		}
 		spsc_produce(node_q[nn->id]);
 	}
 }