diff --git a/firmware/src_simtrace/host_communication.c b/firmware/src_simtrace/host_communication.c
index 6834c4a4..db328d4e 100644
--- a/firmware/src_simtrace/host_communication.c
+++ b/firmware/src_simtrace/host_communication.c
@@ -10,7 +10,12 @@ static void usb_write_cb(uint8_t *arg, uint8_t status, uint32_t transferred,
 {
 	struct req_ctx *rctx = (struct req_ctx *) arg;
 
-	usbep_in_progress[rctx->ep] = 0;
+	TRACE_DEBUG("%s (EP=%u)\r\n", __func__, rctx->ep);
+
+	__disable_irq();
+	usbep_in_progress[rctx->ep]--;
+	__enable_irq();
+	TRACE_DEBUG("%u: in_progress=%d\n", rctx->ep, usbep_in_progress[rctx->ep]);
 
 	if (status != USBD_STATUS_SUCCESS)
 		TRACE_ERROR("%s error, status=%d\n", __func__, status);
@@ -24,15 +29,27 @@ int usb_refill_to_host(struct llist_head *queue, uint32_t ep)
 	struct req_ctx *rctx;
 	int rc;
 
-	if (usbep_in_progress[ep])
+	__disable_irq();
+	if (usbep_in_progress[ep]) {
+		__enable_irq();
 		return 0;
+	}
 
-	if (llist_empty(queue))
+	if (llist_empty(queue)) {
+		__enable_irq();
 		return 0;
+	}
+
+	usbep_in_progress[ep]++;
 
 	rctx = llist_entry(queue->next, struct req_ctx, list);
 	llist_del(&rctx->list);
 
+	__enable_irq();
+
+	TRACE_DEBUG("%u: in_progress=%d\n", ep, usbep_in_progress[ep]);
+	TRACE_DEBUG("%s (EP=%u)\r\n", __func__, ep);
+
 	req_ctx_set_state(rctx, RCTX_S_USB_TX_BUSY);
 	rctx->ep = ep;
 
@@ -41,11 +58,13 @@ int usb_refill_to_host(struct llist_head *queue, uint32_t ep)
 	if (rc != USBD_STATUS_SUCCESS) {
 		TRACE_ERROR("%s error %x\n", __func__, rc);
 		req_ctx_set_state(rctx, RCTX_S_USB_TX_PENDING);
+		__disable_irq();
+		usbep_in_progress[ep]--;
+		__enable_irq();
+		TRACE_DEBUG("%u: in_progress=%d\n", ep, usbep_in_progress[ep]);
 		return 0;
 	}
 
-	usbep_in_progress[ep] = 1;
-
 	return 1;
 }
 
@@ -55,6 +74,8 @@ static void usb_read_cb(uint8_t *arg, uint8_t status, uint32_t transferred,
 	struct req_ctx *rctx = (struct req_ctx *) arg;
 	struct llist_head *queue = (struct llist_head *) usbep_in_progress[rctx->ep];
 
+	TRACE_DEBUG("%s (EP=%u)\r\n", __func__, rctx->ep);
+
 	usbep_in_progress[rctx->ep] = 0;
 
 	if (status != USBD_STATUS_SUCCESS) {
@@ -75,6 +96,8 @@ int usb_refill_from_host(struct llist_head *queue, int ep)
 	if (usbep_in_progress[ep])
 		return 0;
 
+	TRACE_DEBUG("%s (EP=%u)\r\n", __func__, ep);
+
 	rctx = req_ctx_find_get(0, RCTX_S_FREE, RCTX_S_USB_RX_BUSY);
 	rctx->ep = ep;