Operating System Restore does not understand where the truth

Thread scheduling in thread_block method before executing the current thread status update to interrupt to shut up, and then call the function to perform handover schedule, schedule function will be replaced by the current thread, the first thread processor to perform re-pop band from the ready queue, there are now in both cases, if the head of the queue at this time is a new thread, the new thread will break open, so in theory the ready queue of all the threads have the opportunity to processors, but this time head of the queue if the thread is already executed , that the thread is not interrupted open, so there will be a new thread has been put on the execution will not be interrupted interrupted until the end of the thread change, but the end is also a problem, in multi-thread scheduling and thread did not write When will it end