Question

Are the P() and V() operations that can be performed on a semaphore guarantee atomic? Can a semaphore prevent two processes getting into the P()?

Answer 1

Suppose we have a binary semaphore, s, which has the value 1, and two processes simultaneously attempt to execute P on s. Only one of these operations will be able to complete before the next V operation on s; the other process attempting to perform a P operation is suspended.

Taken from my university notes:

We can think if P and V as controlling
access to a resource:

When a process wants to use the
resource, it performs a P operation:
if this succeeds, it decrements the
amount of resource available and the
process continues; if all the
resource is currently in use, the
process has to wait.

When a process is finished with the
resource, it performs a V operation:
if there were processes waiting on the
resource, one of these is woken up;
if there were no waiting processes,
the semaphore is incremented
indicating that there is now more of
the resource free. Note that the
definition of V doesn’t specify which
process is woken up if more than one
process has been suspended on the same
semaphore.

Semaphores can solve both mutual exclusion and condition synchronization problems. So the answer to both your questions is: yes.