Semaphore/ Mutex/ Spinlock

kernel resources that provide synchronization services
synchronization services to prevent multi threads to access shared resrouce at the same time
limit access to shared resource in multi programming

⭐️ Critical Section

program code to access shared data

• if one process is running in Critical Section, other processes cannot access
• However, multiple threads can access the Critical Section at the same time
• thus, need technique such as semaphore, mutex, spinlock

✅ Semaphore

singnaling mechanism: thread can be signaled by another thread
prevent process/thread from accessing critical section
use value

• signaling mechanism
• Wait(P), Signal(V)
  
• machine independent
  
• 해도 돼, 기다려! 시그널을 준다.
• 👍🏻 can be used for mutual exclusion, or counting semaphore

• 👎🏻 resource might be blocked for a long time

• Binary semaphore: 0, 1
  • like mutex
• Counting semaphore: count number of available resource

화장실: critical section
사람들: process/thread

Semaphore에서는
총 화장실 개수, 화장실 빈 칸 개수 알려줌
화장실을 가기 위해서는 빈 칸이 1개 이상이면 갈 수 있고,
갔다 온 뒤에는 빈 칸 개수 +1을 해 줘야 함
⭐️ counting semaphore

✅ Mutex

⭐️ Mutual Exclusion
thread execution time to be independent
use lock(key)

• ⭐️ lock, unlock
  
• kind of binary semaphore
• shared resource 잠궈버리기, 뮤텍스 잠겨 있으면 현재 스레드는 다른 작업 하다가 뮤텍스 해제되면 접근 가능
• only mutual exclusion possible

화장실: critical section
사람들: process/thread

Mutex에서는
화장실 키가 1개 있음
키가 있으면 화장실에 갈 수 있고, 없으면 갈 수 없음
⭐️ key, lock, object to access critical section

⭐️ Lock, unlock

lock: authorized to go into Critical Section
unlock: annouce this process is done with Critical Section

Mutex Algorithm

• Dekker algorithm
• Peterson algorithm
• Backery algorithm

✅ Spinlock

• locking system mechanism
• simply wait in loop until the lock is available
• thread waits in a loop or spin until the lock is available
• 👍🏻 only mutual exclusion possible
• 👍🏻 blocked only for a short period of time

💡 https://www.geeksforgeeks.org/mutex-vs-semaphore/