serial port is hostage of writer thread

Question

I have two real-time threads(linux)

First one is to send bytes over serial port (kernel space)

Second one is to read the same bytes(because i used loop-back device) from serial port.

First thread sends data continuously and asynchronously, however;

second thread cannot open the serial port to read..

I used semaphores, but my problem i guess.. is not the synchronization issue, my problem is serial port still in the hand of first thread.

How can I overcome this issue..

I am open to your suggestions,

Best Regards