Semaphore is synchronization tool to solve critical-section problem, can be used to control access to the critical section for a process or thread. The main disadvantage (same of mutual-exclusion) is requiring busy waiting. It will create problems in a multiprogramming system, where a single CPU is shared among many processes.

Busy waiting wastes CPU cycles.

Deadlock is a situation when two or more processes are waiting indefinitely for an event that can be caused by only one of the waiting processes. The implementation of a semaphore with a waiting queue may result in this situation.