CS 333Introduction to Operating Systems Class 4 –.ppt

CS 333Introduction to Operating Systems Class 4 – Synchronization PrimitivesSemaphores Jonathan Walpole Computer Science Portland State University Using mutual exclusion for critical sections How can we enforce mutual exclusion? What about using locks ? Locks solve the problem of exclusive access to shared data. Acquiring a lock prevents concurrent access Expresses intention to enter critical section Assumption: Each shared data item has an associated lock All threads set the lock before accessing the shared data Every thread releases the lock after it is done Acquiring and releasing locks Acquiring and releasing locks Acquiring and releasing locks Acquiring and releasing locks Acquiring and releasing locks Acquiring and releasing locks Acquiring and releasing locks Acquiring and releasing locks Acquiring and releasing locks Acquiring and releasing locks Acquiring and releasing locks Acquiring and releasing locks Acquiring and releasing locks Acquiring and releasing locks Acquiring and releasing locks Acquiring and releasing locks Mutex locks An abstract data type Used for synchronization and mutual exclusion The mutex is either: Locked (“the lock is held”) Unlocked (“the lock is free”) Mutex lock operations Lock (mutex) Acquire the lock if it is free Otherwise wait until it can be acquired Unlock (mutex) Release the lock If there are waiting threads wake up one of them How to use a mutex? But how can we implement a mutex? Lock and Unlock operations must be atomic ! Can we just set and clear a binary “lock” variable in memory? Many computers have some limited hardware support for setting locks Atomic Test and Set Lock instruction Atomic compare and swap operation These can be used to implement mutex locks Test-and-set-lock instruction (TSL, tset) A lock is a single word variable with two values 0 = FALSE = not locked 1 = TRUE = locked Test-and-set does the following atomically: Get the (old) value Set the lock to TRUE Return the old value If th