Binary semaphore

Question

In the standard code for UP operation,

UP (S)

{

       if(Waiting queue L is empty)   

              S = 1;

      else

               Select a process from waiting queue and wake up();

}

( S is the mutex)

Here, why S=1 is not set in the else part as well?

For eg: if S=0 and many processes are already waiting in the queue, then one other process can still do UP(), make S=1 and enter the critical section right?

ie, shouldn't the "else' part be:

else

{

                S = 1;

               Select a process from waiting queue and wake up();

}

 

Or, is it mandatory that a process can enter the critical section only after letting in, all the processes the got blocked before it?

Answer 1

"Here, why S=1 is not set in the else part as well?" 
You have taken example of Binary Semaphore, S=1 is not set in else part to maintain Mutual Exclusion. Two process can go in CS together because one process is woke up and another process can go because it finds S=1. 

Or, is it mandatory that a process can enter the critical section only after letting in, all the processes the got blocked before it?
Yes! until block queue of a particular binary semaphore becomes empty, S can't be set to 1.
When there are many processes blocked in the queue of a semaphore (each semaphore variable has its own block queue), process wake up means its PCB has been placed in ready queue, whether it will be executed immediately or later it depends on the CPU schedulig.