Atomicity, Mutex, and Locks David Ferry CSCI 3500 – Operating Systems Saint Louis University St. Louis, MO 63103

CSCI 3500 - Operating Systems Critical Sections Recall the problem with race conditions: Suppose x=0 initially: Thread 1 Thread 2 u = x v = x u = u + 1 v = v * 2 x = u x = v A critical section is any piece of code that should not execute simultaneously with another piece of code. CSCI 3500 - Operating Systems

CSCI 3500 - Operating Systems Critical Sections Recall the problem with race conditions: Suppose x=0 initially: Thread 1 Thread 2 u = x v = x u = u + 1 v = v * 2 x = u x = v A critical section is any piece of code that should not execute simultaneously with another piece of code (potentially itself- see push() from last time). CSCI 3500 - Operating Systems

Locks & Mutex (Mutual Exclusion) We need to prevent simultaneous access to critical sections. Something like: mutex m; lock(m); //Do critical section unlock(m); Where lock(m): Proceeds if m is unlocked If not, blocks until m is unlocked CSCI 3500 - Operating Systems

Implementing lock() and unlock()? lock() should block until the mutex variable is unlocked, a naïve implementation: int mutex = 0; lock( mutex ): unlock( mutex ): while(1){ mutex = 0; if( mutex == 0 ){ mutex = 1; break; } CSCI 3500 - Operating Systems

CSCI 3500 - Operating Systems But wait… Another race condition! lock( mutex ) translates to: while(1){ start: if( mutex == 0 ){ load m to register mutex = 1; compare m break; jump if not zero->start } store 1 to memory } What if we could simultaneously test and update the value of the mutex variable? CSCI 3500 - Operating Systems

CSCI 3500 - Operating Systems Atomic Operations An atomic operation is one that is indivisible. From the previous slide, what we want is this: if ( m == 0 ) //Test the value of m m = 1; //Set the value of m If this were atomic, then it looks as though both statements execute simultaneously. The outside world sees the “before” or “after” but never any intermediate states. In particular, a second thread can’t come along and read the value of m after it has already been read but before the assignment m=1 takes place. CSCI 3500 - Operating Systems

CSCI 3500 - Operating Systems Hardware to the Rescue Hardware designers can give us a special atomic test and set instruction. Then: lock(mutex): while(1){ int success = hardware_test_and_set(mutex); if( success ) break; } The lock() functions we use probably look something like this, modulo performance optimizations. CSCI 3500 - Operating Systems

CSCI 3500 - Operating Systems Unsatisfying? Do we have to rely on hardware giving us a special “solve everything” instruction? In fact, mutual exclusion can be accomplished only with regular memory loads and stores (i.e. in software). The first such solution, Dekker’s Algorithm, was published by Edsger Dijkstra in 1968. Leslie Lamport published the first general solution called the Bakery Algorithm in 1974. Other solutions exist since then. Hardware implementations tend to be faster and easier to implement. However, software-only solutions can be useful, such as when portability is important. CSCI 3500 - Operating Systems

Other Atomic Operations We will see other atomic operations in the studios. E.g.: __sync_fetch_and_add(); This function synchronously (atomically) loads a value from memory and adds to it. Thread 1: Thread 2: x++; x++; CSCI 3500 - Operating Systems

Solving Race Conditions Critical sections occur when variables are shared between threads, in particular when shared variables are written to. lock() and unlock(): Can encapsulate any critical section Code/data can be any size Atomic operations: Usually faster, if applicable CSCI 3500 - Operating Systems