bootstrap模式不起作用ajax async false

Question

JS

function get_pcap_count(){
    aid=document.getElementById('AnalysisID').value;
    var base_dir = "<?php echo getcwd(); ?>"+"/pcap";
    $('#myPleaseWait').modal('show');
    $.ajax({
            async:false,
            dataType:'json',
            url:'./get_pcap_count.php',
            type:'POST',
            data:{base_dir:base_dir,aid:aid},
            success:function(data){             
                document.getElementById('pcapcount').innerHTML=data;    
            },
            error:function(request,status,error){
                ;
            }
    });
}

模态

<!-- Modal Start here-->
<div class="modal fade bs-example-modal-sm" id="myPleaseWait" role="dialog" aria-hidden="true" data-backdrop="static" style="position:fixed; top:33% !important;">
    <div class="modal-dialog modal-sm">
        <div class="modal-content">
            <div class="modal-header">
                <h4 class="modal-title">
                    <span class="glyphicon glyphicon-time">
                    </span>Please Wait
                 </h4>
            </div>
            <div class="modal-body">
                <div class="progress">
                    <div class="progress-bar progress-bar-info
                    progress-bar-striped active"
                    style="width: 100%">
                    </div>
                </div>
            </div>
        </div>
    </div>
</div>
<!-- Modal ends Here -->

bootstrap模式不起作用ajax async false。

但是bootstrap模式可以很好地运行ajax async true。

我必须使用async false，因为防止重复。

那么如何在async中显示模态？