Advanced Operating Systems - Lecture 12: Readers/Writers problem. This lecture will cover the following: readers/writers problem; solving readers/writers problem using condition variables; pros and cons of the solution; duality of synchronization primitives; implementing condition variables using semaphores as building blocks; thread safety and reentrant functions;...
CS703 Advanced Operating Systems By Mr Farhan Zaidi Lecture No. 12 Overview of today’s lecture Readers/writers problem Solving readers/writers problem using condition variables Pros and cons of the solution Duality of synchronization primitives Implementing condition variables using semaphores as building blocks Thread safety and reentrant functions Ways to solve thread un-safety problem of library functions Thread un-safe functions in C library Recap of lecture Readers/Writers (2) Constraints Readers can access database when no writers (Condition okToRead) Writers can access database when no readers or writers (Condition okToWrite) Only one thread manipulates state variables at a time Readers/Writers(3) Basic structure of solution Reader wait until no writers access database check out wake up waiting writer Writer wait until no readers or writers access database check out wake up waiting readers or writer State variables: # of active readers AR = # of active writers AW = # of waiting readers WR = # of waiting writers WW = Condition okToRead = NIL Condition okToWrite = NIL Lock lock = FREE Readers/Writers (4) Code: Reader() { lock.Acquire(); while ((AW + WW) > 0) { // check if safe to read // if any writers, wait WR++; okToRead.Wait(&lock); WR ; } AR++; lock.Release(); Access DB lock.Acquire(); AR ; If (AR == && WW > 0)//if no other readers still // active, wake up writer okToWrite.Signal(&lock); lock.Release(); } Readers/Writers (5) Writer() { // symmetrical lock.Acquire(); while ((AW + AR) > 0) { // check if safe to write // if any readers or writers, wait WW++; okToWrite->Wait(&lock); WW ; } AW++; lock.Release(); Access DB // check out lock.Acquire(); AW ; if (WW > 0) // give priority to other writers okToWrite->Signal(&lock); else if (WR > 0) okToRead->Broadcast(&lock); lock.Release(); } Questions Can readers or writers starve? Who and Why? Why does checkRead need a while? semaphores and monitors Illustrate the differences by considering: can we build monitors out of semaphores? After all, semaphores provide atomic operations and queueing Does this work? Wait() { semaphore - > P(); } Signal() { semaphore - > V(); } Condition variables only work inside of a lock Does this work? Wait(Lock *lock) { lock->Release(); Semaphore - > P(); Lock - > Acquire(); } Signal() { Semaphore - > V(); } semaphores and monitors(2) What if thread signals and no one is waiting? No op What if thread later waits? Thread waits What if thread V's and no one is waiting? Increment What if thread later does P? Decrement and continue In other words, P + V are commutative result is the same no matter what order they occur Condition variables are NOT commutative That's why they must be in a critical section need to access state variables to their job Does this fix the problem? Signal() { if semaphore queue is not empty semaphore->V(); } For one, not legal to look at contents of semaphore queue But also: race condition signaller can slip in after lock is released, and before wait Then waiter never wakes up! Need to release lock and go to sleep atomically Is it possible to implement condition variables using semaphores? Yes!!! Semaphore mutex = 1; // This lock is outside of the condition object Condition { Semaphore lock = 1; Seamphore waitSem = 0; Int numWaiters = 0; } wait(cond, mutex) { P(cond.lock); cond.numWaiters++; V(cond.lock); V(mutex); P(cond.waitSem); P(cond.lock); cond.numWaiters - -; P(mutex); } signal (cond, mutex) { P(cond.lock); if (cond.numWaiters > 0) { V(cond waitSem); } V(cond.lock); } V(cond.lock); Thread Safety Functions called from a thread must be thread-safe We identify four (non-disjoint) classes of thread-unsafe functions: Class 1: Failing to protect shared variables Class 2: Relying on persistent state across invocations Class 3: Returning a pointer to a static variable Class 4: Calling thread-unsafe functions ThreadUnsafe Functions Class 1: Failing to protect shared variables Fix: Use P and V semaphore operations Issue: Synchronization operations will slow down code ThreadUnsafe Functions (cont) Class 3: Returning a ptr to a static variable Fixes: Rewrite code so caller passes pointer to struct Issue: Requires changes in caller and callee Lock-and-copy Issue: Requires only simple changes in caller (and none in callee) However, caller must free memory struct hostent *gethostbyname(char name) { static struct hostent h; return &h; } hostp = Malloc( )); gethostbyname_r(name, hostp); struct hostent *gethostbyname_ts(char *p) { struct hostent *q = Malloc( ); P(&mutex); /* lock */ p = gethostbyname(name); *q = *p; /* copy */ V(&mutex); return q; } ThreadUnsafe Functions Class 4: Calling thread-unsafe functions Calling one thread-unsafe function makes an entire function thread-unsafe Fix: Modify the function so it calls only thread-safe functions Reentrant Functions A function is reentrant iff it accesses NO shared variables when called from multiple threads Reentrant functions are a proper subset of the set of thread-safe functions Thread-safe functions Reentrant functions Thread-unsafe functions ThreadSafe Library Functions All functions in the Standard C Library are thread-safe Examples: malloc, free, printf, scanf Most Unix system calls are thread-safe, with a few exceptions: Thread-unsafe function Class asctime ctime gethostbyaddr gethostbyname inet_ntoa localtime rand Reentrant version asctime_r ctime_r gethostbyaddr_r gethostbyname_r (none) localtime_r rand_r ... { semaphore - > P(); } Signal() { semaphore - > V(); } Condition variables only work inside of a lock Does this work? Wait(Lock *lock) { lock->Release(); Semaphore - > P(); Lock - > Acquire();... Class 4: Calling thread-unsafe functions Calling one thread-unsafe function makes an entire function thread-unsafe Fix: Modify the function so it calls only thread-safe functions Reentrant Functions... set of thread-safe functions Thread-safe functions Reentrant functions Thread-unsafe functions ThreadSafe Library Functions All functions in the Standard C Library are thread-safe Examples: