Bluetooth: Controller: Fix Tx Ack FIFO index corruption under race

Fix Tx Ack FIFO's first index being advanced beyond a
recorded ack_last value in a node_rx when under race
between ll_rx_get() being pre-empted while executing the
`tx_cmplt_get()` and a call to `ll_rx_put()` in an
interrupt service routine.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>

Author: cvinayak

subsys/bluetooth/controller/ll_sw/ull.c

@@ -968,8 +968,8 @@ void ll_reset(void)
 uint8_t ll_rx_get(void **node_rx, uint16_t *handle)
 {
 	struct node_rx_pdu *rx;
-	memq_link_t *link;
 	uint8_t cmplt = 0U;
+	memq_link_t *link;
 
 #if defined(CONFIG_BT_CONN) || \
 	(defined(CONFIG_BT_OBSERVER) && defined(CONFIG_BT_CTLR_ADV_EXT)) || \
@@ -984,6 +984,20 @@ uint8_t ll_rx_get(void **node_rx, uint16_t *handle)
 
 	*node_rx = NULL;
 
+#if defined(CONFIG_BT_CONN) || defined(CONFIG_BT_CTLR_ADV_ISO)
+	/* Save the tx_ack FIFO's last index to avoid the value being changed if there were no
+	 * Rx PDUs and we were pre-empted before calling `tx_cmplt_get()`, that may advance the
+	 * first index beyond ack_last value recorded in node_rx enqueued by `ll_rx_put()` call
+	 * when we are in the `else` clause below.
+	 */
+	uint8_t tx_ack_last = mfifo_fifo_tx_ack.l;
+
+	/* Ensure that the value is fetched before call to memq_peek, i.e. compiler shall not
+	 * reorder memory write before above read.
+	 */
+	cpu_dmb();
+#endif /* CONFIG_BT_CONN || CONFIG_BT_CTLR_ADV_ISO */
+
 	link = memq_peek(memq_ll_rx.head, memq_ll_rx.tail, (void **)&rx);
 	if (link) {
 #if defined(CONFIG_BT_CONN) || defined(CONFIG_BT_CTLR_ADV_ISO)
@@ -1064,8 +1078,11 @@ uint8_t ll_rx_get(void **node_rx, uint16_t *handle)
 #if defined(CONFIG_BT_CONN) || defined(CONFIG_BT_CTLR_ADV_ISO)
 		}
 	} else {
-		cmplt = tx_cmplt_get(handle, &mfifo_fifo_tx_ack.f,
-				     mfifo_fifo_tx_ack.l);
+		/* Use the saved ack last value, before call was done to memq_peek, to ensure we
+		 * do not advance the first index `f` beyond the value that was the last index `l`
+		 * when memq_peek was called.
+		 */
+		cmplt = tx_cmplt_get(handle, &mfifo_fifo_tx_ack.f, tx_ack_last);
 #endif /* CONFIG_BT_CONN || CONFIG_BT_CTLR_ADV_ISO */
 	}