Mutex Introduction

• MUTual EXclusion object
• We can use a mutex to implement locking
• A mutex has two states
  • "locked"
  • "unlocked"

Mutual Exclusion

• Exclusion
  • The mutex is used to exclude threads from the critical section
• Mutual
  • The threads agree to respect the mutex
• Locking
  • If the mutex is unlocked, a thread can enter the critical section
  • If the mutex is locked, no thread can enter until it becomes unlocked
  • A thread locks the mutex when it enters the critical section
  • A thread unlocks the mutex when it leaves the critical section

Thread Synchronization with Mutex

• Some threads A, B, C, ... wish to enter a critical section

  • Thread A locks the mutex
  • Thread A enters the critical section
  • Threads B, C... wait until they can lock the mutex
  • Thread A leaves the critical section
  • Thread A unlocks the mutex
  • One of threads B, C, ... can now lock the mutex and enter the critical section

• The threads are synchronized

  • They cannot interleave when they execute in the critical section
  • There is no data race

• Unlocking a mutex "publishes" any changes

  • Thread A modifies shared data
  • The new value is now available to other threads
  • It will be seen by the next thread which accesses the shared data

Acquire-Release Semantics

• A thread locks a mutex

  • It "acquires" exclusive access to the critical section

• The thread unlocks the mutex

  • It "releases" exclusive access to the critical section
  • It also "releases" the result of any modifications
  • The next thread that locks the mutex will acquire these results

• These acquire-release semantics impose ordering on the threads

  • There is no data race
  • The shared data is always in a consistent state