diff --git a/libusb/os/linux_usbfs.c b/libusb/os/linux_usbfs.c
index c84e805..2b81189 100644
--- a/libusb/os/linux_usbfs.c
+++ b/libusb/os/linux_usbfs.c
@@ -1860,15 +1860,22 @@ static void op_clear_transfer_priv(struct usbi_transfer *itransfer)
 		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
 	struct linux_transfer_priv *tpriv = usbi_transfer_get_os_priv(itransfer);
 
+	/* urbs can be freed also in submit_transfer so lock mutex first */
 	switch (transfer->type) {
 	case LIBUSB_TRANSFER_TYPE_CONTROL:
 	case LIBUSB_TRANSFER_TYPE_BULK:
 	case LIBUSB_TRANSFER_TYPE_INTERRUPT:
-		free(tpriv->urbs);
+		usbi_mutex_lock(&itransfer->lock);
+		if (tpriv->urbs)
+			free(tpriv->urbs);
 		tpriv->urbs = NULL;
+		usbi_mutex_unlock(&itransfer->lock);
 		break;
 	case LIBUSB_TRANSFER_TYPE_ISOCHRONOUS:
-		free_iso_urbs(tpriv);
+		usbi_mutex_lock(&itransfer->lock);
+		if (tpriv->iso_urbs)
+			free_iso_urbs(tpriv);
+		usbi_mutex_unlock(&itransfer->lock);
 		break;
 	default:
 		usbi_err(TRANSFER_CTX(transfer),