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D u k e S y s t e m s Threads and Synchronization Jeff Chase Duke University.

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1 D u k e S y s t e m s Threads and Synchronization Jeff Chase Duke University

2 A process can have multiple threads volatile int counter = 0; int loops; void *worker(void *arg) { int i; for (i = 0; i < loops; i++) { counter++; } pthread_exit(NULL); } int main(int argc, char *argv[]) { if (argc != 2) { fprintf(stderr, "usage: threads \n"); exit(1); } loops = atoi(argv[1]); pthread_t p1, p2; printf("Initial value : %d\n", counter); pthread_create(&p1, NULL, worker, NULL); pthread_create(&p2, NULL, worker, NULL); pthread_join(p1, NULL); pthread_join(p2, NULL); printf("Final value : %d\n", counter); return 0; } data [code from OSTEP]

3 Threads A thread is a stream of control. – defined by CPU register context (PC, SP, …) – Note: process context is thread context plus protected registers defining current VAS, e.g., ASID or page table base register(s). – Generally context is the register values and referenced memory state (stack, page tables) Multiple threads can execute independently: – They can run in parallel on multiple cores... physical concurrency – …or arbitrarily interleaved on a single core. logical concurrency – Each thread must have its own stack.

4 Threads and the kernel Modern operating systems have multi- threaded processes. A program starts with one main thread, but once running it may create more threads. Threads may enter the kernel (e.g., syscall). Threads are known to the kernel and have separate kernel stacks, so they can block independently in the kernel. – Kernel has syscalls to create threads (e.g., Linux clone). Implementations vary. – This model applies to Linux, MacOS-X, Windows, Android, and pthreads or Java on those systems. data trap fault resume user mode user space kernel mode kernel space process threads VAS

5 Portrait of a thread ucontext_t name/status etc 0xdeadbeef Stack Thread operations (parent) a rough sketch: t = create(); t.start(proc, argv); t.alert(); (optional) result = t.join(); Self operations (child) a rough sketch: exit(result); t = self(); setdata(ptr); ptr = selfdata(); alertwait(); (optional) Details vary. Thread Control Block (TCB) Heuristic fencepost: try to detect stack overflow errors Storage for context (register values) when thread is not running.

6 A thread Program kernel stack user stack User TCB kernel TCB active ready or running blocked wait sleep wait wakeup signal When a thread is blocked its TCB is placed on a sleep queue of threads waiting for a specific wakeup event. This slide applies to the process abstraction too, or, more precisely, to the main thread of a process.

7 Java threads: the basics class RunnableTask implements Runnable { public RunnableTask(…) { // save any arguments or input for the task (optional) } public void run() { // do task: your code here } … RunnableTask task = new RunnableTask(); Thread t1= new Thread(task, "thread1"); t1.start(); …

8 Java threads: the basics public void MyThread extends Thread { public void run() { // do task: your code here } … Thread t1 = new MyThread(); t1.start(); If you prefer, you may extend the Java Thread class.

9 CPU Scheduling 101 The OS scheduler makes a sequence of moves. – Next move: if a CPU core is idle, pick a ready thread from the ready pool and dispatch it (run it). – Schedulers choice is nondeterministic – Scheduler and machine determine the interleaving of execution (a schedule). Wakeup GetNextToRun SWITCH() ready pool blocked threads If timer expires, or wait/yield/terminate

10 Non-determinism and ordering Time Thread A Thread B Thread C Global ordering Why do we care about the global ordering? Might have dependencies between events Different orderings can produce different results Why is this ordering unpredictable? Cant predict how fast processors will run

11 Non-determinism example y=10; Thread A: x = y+1; Thread B: y = y*2; Possible results? A goes first: x = 11 and y = 20 B goes first: y = 20 and x = 21 Variable y is shared between threads.

12 Another example Two threads (A and B) A tries to increment i B tries to decrement i i = 0; Thread A: while (i < 10){ i++; } print A done. Thread B: while (i > -10){ i--; } print B done.

13 Example continued Who wins? Does someone have to win? Thread A: i = 0; while (i < 10){ i++; } print A done. Thread B: i = 0; while (i > -10){ i--; } print B done.

14 Two threads sharing a CPU reality concept context switch

15 Resource Trajectory Graphs Resource trajectory graphs (RTG) depict the random walk through the space of possible program states. RTG is useful to depict all possible executions of multiple threads. I draw them for only two threads because slides are two-dimensional. RTG for N threads is N-dimensional. Thread i advances along axis i. Each point represents one state in the set of all possible system states. Cross-product of the possible states of all threads in the system SnSn SoSo SmSm

16 Resource Trajectory Graphs This RTG depicts a schedule within the space of possible schedules for a simple program of two threads sharing one core. Blue advances along the y-axis. Purple advances along the x-axis. The scheduler chooses the path (schedule, event order, or interleaving). The diagonal is an idealized parallel execution (two cores). Every schedule starts here. EXIT Every schedule ends here. context switch From the point of view of the program, the chosen path is nondeterministic.

17 A race start x=x+1 This is a valid schedule. But the schedule interleaves the executions of x = x+ 1 in the two threads. The variable x is shared (like the counter in the pthreads example). This schedule can corrupt the value of the shared variable x, causing the program to execute incorrectly. This is an example of a race: the behavior of the program depends on the schedule, and some schedules yield incorrect results.

18 Reading Between the Lines of C load add store load add store Two executions of this code, so: x is incremented by two. loadx, R2; load global variable x addR2, 1, R2; increment: x = x + 1 storeR2, x; store global variable x Two threads execute this code section. x is a shared variable.

19 Interleaving matters loadx, R2; load global variable x addR2, 1, R2; increment: x = x + 1 storeR2, x; store global variable x load add store load add store In this schedule, x is incremented only once: last writer wins. The program breaks under this schedule. This bug is a race. Two threads execute this code section. x is a shared variable. X

20 Concurrency control Each thread executes a sequence of instructions, but their sequences may be arbitrarily interleaved. – E.g., from the point of view of loads/stores on memory. Each possible execution order is a schedule. It is the programs responsibility to exclude schedules that lead to incorrect behavior. It is called synchronization or concurrency control. The scheduler (and the machine) select the execution order of threads

21 This is not a game But we can think of it as a game. 1.You write your program. 2.The game begins when you submit your program to your adversary: the scheduler. 3.The scheduler chooses all the moves while you watch. 4.Your program may constrain the set of legal moves. 5.The scheduler searches for a legal schedule that breaks your program. 6.If it succeeds, then you lose (your program has a race). 7.You win by not losing. x=x+1

22 The need for mutual exclusion The program may fail if the schedule enters the grey box (i.e., if two threads execute the critical section concurrently). The two threads must not both operate on the shared global x at the same time. x=x+1 x=???

23 A Lock or Mutex Locks are the basic tools to enforce mutual exclusion in conflicting critical sections. A lock is a special data item in memory. API methods: Acquire and Release. Also called Lock() and Unlock(). Threads pair calls to Acquire and Release. Acquire upon entering a critical section. Release upon leaving a critical section. Between Acquire/Release, the thread holds the lock. Acquire does not pass until any previous holder releases. Waiting locks can spin (a spinlock) or block (a mutex). A A R R

24 Definition of a lock (mutex) Acquire + release ops on L are strictly paired. – After acquire completes, the caller holds (owns) the lock L until the matching release. Acquire + release pairs on each L are ordered. – Total order: each lock L has at most one holder at any given time. – That property is mutual exclusion; L is a mutex.

25 OSTEP pthread example (2) pthread_mutex_t m; volatile int counter = 0; int loops; void *worker(void *arg) { int i; for (i = 0; i < loops; i++) { Pthread_mutex_lock(&m); counter++; Pthread_mutex_unlock(&m); } pthread_exit(NULL); } Lock it down. load add store load add store A A R R

26 Portrait of a Lock in Motion A A R R The program may fail if it enters the grey box. A lock (mutex) prevents the schedule from ever entering the grey box, ever: both threads would have to hold the same lock at the same time, and locks dont allow that. x=x+1 x=???

27 Handing off a lock First I go. Then you go. release acquire Handoff The nth release, followed by the (n+1)th acquire serialized (one after the other)

28 Mutual exclusion in Java Mutexes are built in to every Java object. – no separate classes Every Java object is/has a monitor. – At most one thread may own a monitor at any given time. A thread becomes owner of an objects monitor by – executing an object method declared as synchronized – executing a block that is synchronized on the object public void increment() { synchronized(this) { x = x + 1; } public synchronized void increment() { x = x + 1; }

29 New Problem: Ping-Pong void PingPong() { while(not done) { … if (blue) switch to purple; if (purple) switch to blue; }

30 Ping-Pong with Mutexes? void PingPong() { while(not done) { Mx->Acquire(); … Mx->Release(); } ???

31 Mutexes dont work for ping-pong

32 Condition variables A condition variable (CV) is an object with an API. – wait: block until the condition becomes true Not to be confused with Unix wait* system call – signal (also called notify): signal that the condition is true Wake up one waiter. Every CV is bound to exactly one mutex, which is necessary for safe use of the CV. – holding the mutex in the monitor A mutex may have any number of CVs bound to it. CVs also define a broadcast (notifyAll) primitive. – Signal all waiters.

33 Condition variable operations wait (){ release lock put thread on wait queue go to sleep // after wake up acquire lock } signal (){ wakeup one waiter (if any) } broadcast (){ wakeup all waiters (if any) } Atomic Lock always held Lock usually held Lock always held

34 Ping-Pong using a condition variable void PingPong() { mx->Acquire(); while(not done) { … cv->Signal(); cv->Wait(); } mx->Release(); }

35 Lab #1

36 OSTEP pthread example (1) volatile int counter = 0; int loops; void *worker(void *arg) { int i; for (i = 0; i < loops; i++) { counter++; } pthread_exit(NULL); } int main(int argc, char *argv[]) { if (argc != 2) { fprintf(stderr, "usage: threads \n"); exit(1); } loops = atoi(argv[1]); pthread_t p1, p2; printf("Initial value : %d\n", counter); pthread_create(&p1, NULL, worker, NULL); pthread_create(&p2, NULL, worker, NULL); pthread_join(p1, NULL); pthread_join(p2, NULL); printf("Final value : %d\n", counter); return 0; } data

37 Threads on cores A A R R load add store jmp load add store jmp load add store jmp load add store jmp load add store jmp load add store jmp int x; worker() { while (1) {x++}; } load add store jmp

38 Interleaving matters loadx, R2; load global variable x addR2, 1, R2; increment: x = x + 1 storeR2, x; store global variable x load add store load add store In this schedule, x is incremented only once: last writer wins. The program breaks under this schedule. This bug is a race. Two threads execute this code section. x is a shared variable. X

39 Lock it down start context switch A R x=x+1 A R Use a lock (mutex) to synchronize access to a data structure that is shared by multiple threads. A thread acquires (locks) the designated mutex before operating on a given piece of shared data. The thread holds the mutex. At most one thread can hold a given mutex at a time (mutual exclusion). Thread releases (unlocks) the mutex when done. If another thread is waiting to acquire, then it wakes. The mutex bars entry to the grey box: the threads cannot both hold the mutex.

40 Spinlock: a first try int s = 0; lock() { while (s == 1) {}; ASSERT (s == 0); s = 1; } unlock (); ASSERT(s == 1); s = 0; } Busy-wait until lock is free. Global spinlock variable Spinlocks provide mutual exclusion among cores without blocking. Spinlocks are useful for lightly contended critical sections where there is no risk that a thread is preempted while it is holding the lock, i.e., in the lowest levels of the kernel.

41 Spinlock: what went wrong int s = 0; lock() { while (s == 1) {}; s = 1; } unlock (); s = 0; } Race to acquire. Two (or more) cores see s == 0.

42 We need an atomic toehold To implement safe mutual exclusion, we need support for some sort of magic toehold for synchronization. – The lock primitives themselves have critical sections to test and/or set the lock flags. Safe mutual exclusion on multicore systems requires some hardware support: atomic instructions – Examples: test-and-set, compare-and-swap, fetch-and-add. – These instructions perform an atomic read-modify-write of a memory location. We use them to implement locks. – If we have any of those, we can build higher-level synchronization objects like monitors or semaphores. – Note: we also must be careful of interrupt handlers…. – They are expensive, but necessary.

43 Atomic instructions: Test-and-Set Spinlock::Acquire () { while(held); held = 1; } Wrong load4(SP), R2; load this busywait: load 4(R2), R3; load held flag bnzR3, busywait; spin if held wasnt zero store #1, 4(R2); held = 1 Right load4(SP), R2; load this busywait: tsl4(R2), R3; test-and-set this->held bnzR3, busywait; spin if held wasnt zero load test store load test store Solution: TSL atomically sets the flag and leaves the old value in a register. Problem: interleaved load/test/store. One example: tsl test-and-set-lock (from an old machine) (bnz means branch if not zero)

44 Threads on cores: with locking load add store jmp tsl L bnz tsl L bnz tsl L bnz tsl L bnz tsl L bnz tsl L bnz load add store zero L jmp int x; worker() while (1) { acquire L; x++; release L;}; } tsl L bnz tsl L bnz zero L tsl L load add store zero L tsl L bnz jmp

45 Threads on cores: with locking A A R R load add store jmp tsl L bnz load add store zero L jmp int x; worker() while (1) { acquire L; x++; release L;}; } tsl L bnz tsl L zero L tsl L atomic spin

46 Spinlock: IA32 Spin_Lock: CMP lockvar, 0 ;Check if lock is free JE Get_Lock PAUSE ; Short delay JMP Spin_Lock Get_Lock: MOV EAX, 1 XCHG EAX, lockvar ; Try to get lock CMP EAX, 0 ; Test if successful JNE Spin_Lock Atomic exchange to ensure safe acquire of an uncontended lock. Idle the core for a contended lock. XCHG is a variant of compare-and-swap: compare x to value in memory location y; if x != *y then exchange x and *y. Determine success/failure from subsequent value of x.

47 Locking and blocking running readyblocked sleep STOP wait wakeup dispatch If thread T attempts to acquire a lock that is busy (held), T must spin and/or block (sleep) until the lock is free. By sleeping, T frees up the core for some other use. Just sitting and spinning is wasteful! Note: H is the lock holder when T attempts to acquire the lock. yield preempt A A R R HT

48 Sleeping in the kernel syscall trapsfaults interrupts sleep queueready queue Any trap or fault handler may suspend (sleep) the current thread, leaving its state (call frames) on its kernel stack and a saved context in its TCB. A later event/action may wakeup the thread.

49 Locking and blocking running readyblocked sleep STOP wait wakeup dispatch T enters the kernel (via syscall) to block. Suppose T is sleeping in the kernel to wait for a contended lock (mutex). When the lock holder H releases, H enters the kernel (via syscall) to wakeup a waiting thread (e.g., T). Note: H can block too, perhaps for some other resource! H doesnt implicitly release the lock just because it blocks. Many students get that idea somehow. yield preempt A A R R HT

50 Blocking kernel TCB active ready or running blocked wait sleep wait wakeup signal When a thread is blocked on a synchronization object (e.g., a mutex or CV) its TCB is placed on a sleep queue of threads waiting for an event on that object. This slide applies to the process abstraction too, or, more precisely, to the main thread of a process. sleep queueready queue

51 Synchronization objects OS kernel API offers multiple ways for threads to block and wait for some event. Details vary, but in general they wait for a specific event on some kernel object: a synchronization object. – I/O completion – wait*() for child process to exit – blocking read/write on a producer/consumer pipe – message arrival on a network channel – sleep queue for a mutex, CV, or semaphore, e.g., Linux futex – get next event/request on a poll set – wait for a timer to expire

52 Windows synchronization objects They all enter a signaled state on some event, and revert to an unsignaled state after some reset condition. Threads block on an unsignaled object, and wakeup (resume) when it is signaled.

53 Andrew Birrell Bob Taylor

54 TYPE Thread; TYPE Forkee = PROCEDURE(REFANY): REFANY; PROCEDURE Fork(proc: Forkee; arg: REFANY): Thread; PROCEDURE Join(thread: Thread): REFANY; VAR t: Thread; t := Fork(a, x); p := b(y); q := Join(t); TYPE Condition; PROCEDURE Wait(m: Mutex; c: Condition); PROCEDURE Signal(c: Condition); PROCEDURE Broadcast(c: Condition);

55 Debugging non-determinism Requires worst-case reasoning Eliminate all ways for program to break Debugging is hard Cant test all possible interleavings Bugs may only happen sometimes Heisenbug Re-running program may make the bug disappear Doesnt mean it isnt still there!

56 Example: event/request queue Incoming event queue worker loop Handle one event, blocking as necessary. When handler is complete, return to worker pool. We can use a mutex to protect a shared event queue. Lock it down. dispatch threads waiting for event But how will worker threads wait on an empty queue? How to wait for arrival of the next event? We need suitable primitives to wait (block) for a condition and notify when it is satisfied. We discussed this structure for a multi-threaded server. handler

57 Example: event/request queue Incoming event queue worker loop Handle one event, blocking as necessary. When handler is complete, return to worker pool. We can synchronize an event queue with a mutex/CV pair. Protect the event queue data structure itself with the mutex. dispatch threads waiting on CV Workers wait on the CV for next event if the event queue is empty. Signal the CV when a new event arrives. This is a producer/consumer problem. handler

58 Java uses mutexes and CVs public class Object { void notify(); /* signal */ void notifyAll(); /* broadcast */ void wait(); void wait(long timeout); } public class PingPong extends Object { public synchronized void PingPong() { while(true) { notify(); wait(); } Every Java object has a monitor and condition variable (CV) built in. There is no separate mutex class or CV class. A thread must own an objects monitor (synchronized) to call wait/notify, else the method raises an IllegalMonitorStateException. Wait(*) waits until the timeout elapses or another thread notifies.

59 Ping-Pong using a condition variable wait notify (signal) wait signal (notify) wait signal public synchronized void PingPong() { while(true) { notify(); wait(); } Interchangeable lingo synchronized == mutex == lock monitor == mutex+CV notify == signal Suppose blue gets the mutex first: its notify is a no-op. waiting for signal cannot acquire mutex

60 Roots: monitors P1() P2() P3() P4() state ready to enter blocked wait() (enter) signal() A monitor is a module in which execution is serialized. A module is a set of procedures with some private state. [Brinch Hansen 1973] [C.A.R. Hoare 1974] Java synchronized just allows finer control over the entry/exit points. Also, each Java object is its own module: objects of a Java class share methods of the class but have private state and a private monitor. At most one thread runs in the monitor at a time. Other threads wait until the monitor is free.

61 Monitors and mutexes are equivalent Entry to a monitor (e.g., a Java synchronized block) is equivalent to Acquire of an associated mutex. – Lock on entry Exit of a monitor is equivalent to Release. – Unlock on exit (or at least return the key…) Note: exit/release is implicit and automatic if the thread exits monitored code by a Java exception. – Much less error-prone then explicit release

62 Monitors and mutexes are equivalent Well: mutexes are more flexible because we can choose which mutex controls a given piece of state. – E.g., in Java we can use one objects monitor to control access to state in some other object. – Perfectly legal! So monitors in Java are more properly thought of as mutexes. Caution: this flexibility is also more dangerous! – It violates modularity: can code know what locks are held by the thread that is executing it? – Nested locks may cause deadlock (later). Keep your locking scheme simple and local! – Java ensures that each Acquire/Release pair (synchronized block) is contained within a method, which is good practice.

63 Using monitors/mutexes P1() P2() P3() P4() state ready to enter blocked wait() (enter) signal() Each monitor/mutex protects specific data structures (state) in the program. Threads hold the mutex when operating on that state. Threads hold the mutex when transitioning the structures from one consistent state to another, and restore the invariants before releasing the mutex. The state is consistent iff certain well-defined invariant conditions are true. A condition is a logical predicate over the state. Example invariant condition E.g.: suppose the state has a doubly linked list. Then for any element e either e.next is null or e.next.prev == e.

64 Monitor wait/signal P1() P2() P3() P4() state ready to enter waiting (blocked) wait() (enter) signal() At most one thread runs in the monitor at a time. wait() signal() A thread may wait (sleep) in the monitor, exiting the monitor. A thread may signal in the monitor. Signal means: wake one waiting thread, if there is one, else do nothing. The awakened thread returns from its wait and reenters the monitor. We need a way for a thread to wait for some condition to become true, e.g., until another thread runs and/or changes the state somehow.

65 Monitor wait/signal P1() P2() P3() P4() state ready to enter waiting (blocked) wait (enter) signal At most one thread runs in the monitor at a time. wait() signal() Two choices: yes or no. If yes, what happens to the thread that called signal within the monitor? Does it just hang there? They cant both be in the monitor. If no, cant other threads get into the monitor first and change the state, causing the condition to become false again? Design question: when a waiting thread is awakened by signal, must it start running immediately? Back in the monitor, where it called wait? ???

66 Mesa semantics P1() P2() P3() P4() state ready to enter waiting (blocked) wait (enter) signal wait() signal() Mesa semantics: no. An awakened waiter gets back in line. The signal caller keeps the monitor. So, cant other threads get into the monitor first and change the state, causing the condition to become false again? Yes. So the waiter must recheck the condition: Loop before you leap. Design question: when a waiting thread is awakened by signal, must it start running immediately? Back in the monitor, where it called wait? ready to (re)enter

67 Alternative: Hoare semantics As originally defined in the 1960s, monitors chose yes: Hoare semantics. Signal suspends; awakened waiter gets the monitor. Monitors with Hoare semantics might be easier to program, somebody might think. Maybe. I suppose. But monitors with Hoare semantics are difficult to implement efficiently on multiprocessors. Birrell et. al. determined this when they built monitors for the Mesa programming language in the 1970s. So they changed the rules: Mesa semantics. Java uses Mesa semantics. Everybody uses Mesa semantics. Hoare semantics are of historical interest only. Loop before you leap!

68 Condition variables are equivalent A condition variable (CV) is an object with an API. A CV implements the behavior of monitor conditions. – interface to a CV: wait and signal (also called notify) Every CV is bound to exactly one mutex, which is necessary for safe use of the CV. – holding the mutex in the monitor A mutex may have any number of CVs bound to it. – (But not in Java: only one CV per mutex in Java.) CVs also define a broadcast (notifyAll) primitive. – Signal all waiters.

69 Producer-consumer problem Pass elements through a bounded-size shared buffer Producer puts in (must wait when full) Consumer takes out (must wait when empty) Synchronize access to buffer Elements pass through in order Examples Unix pipes: cpp | cc1 | cc2 | as Network packet queues Server worker threads receiving requests Feeding events to an event-driven program

70 Example: the soda/HFCS machine Vending machine (buffer) Soda drinker (consumer) Delivery person (producer)

71 Producer-consumer code producer () { add one soda to machine } consumer () { take a soda from machine }

72 Solving producer-consumer 1.What are the variables/shared state? Soda machine buffer Number of sodas in machine ( MaxSodas) 2.Locks? 1 to protect all shared state (sodaLock) 3.Mutual exclusion? Only one thread can manipulate machine at a time 4.Ordering constraints? Consumer must wait if machine is empty (CV hasSoda) Producer must wait if machine is full (CV hasRoom)

73 Producer-consumer code producer () { lock (sodaLock) while(numSodas==MaxSodas){ wait (sodaLock, hasRoom) } add one soda to machine signal (hasSoda) unlock (sodaLock) } consumer () { lock (sodaLock) while (numSodas == 0) { wait (sodaLock,hasSoda) } take a soda from machine signal (hasRoom) unlock (sodaLock) } MxCV1MxCV2 CV1 CV2

74 Producer-consumer code producer () { lock (sodaLock) while(numSodas==MaxSodas){ wait (sodaLock, hasRoom) } fill machine with soda broadcast(hasSoda) unlock (sodaLock) } consumer () { lock (sodaLock) while (numSodas == 0) { wait (sodaLock,hasSoda) } take a soda from machine signal(hasRoom) unlock (sodaLock) } The signal should be a broadcast if the producer can produce more than one resource, and there are multiple consumers. lpcox slide edited by chase

75 C1/C2 user pseudocode while(until EOF) { read(0, buf, count); compute/transform data in buf; write(1, buf, count); } Pipes AGAIN C1 C2 stdin stdout stdin P Kernel-space pseudocode ???

76 Pipes C1 C2 stdin stdout stdin Kernel-space pseudocode System call internals to read/write N bytes for buffer size B. read(buf, N) { for (i = 0; i++; i

77 Pipes C1 C2 stdin stdout stdin read(buf, N) { pipeMx.lock(); for (i = 0; i++; i

78 Pipes C1 C2 stdin stdout stdin read(buf, N) { readerMx.lock(); pipeMx.lock(); for (i = 0; i++; i

79 Locking a critical section mx->Acquire(); x = x + 1; mx->Release(); mx->Acquire(); x = x + 1; mx->Release(); load add store load add store Holding a shared mutex prevents competing threads from entering a critical section protected by the shared mutex (monitor). At most one thread runs in the critical section at a time. A A R R The threads may run the critical section in either order, but the schedule can never enter the grey region where both threads execute the section at the same time. x=x+1

80 Locking a critical section mx->Acquire(); x = x + 1; mx->Release(); mx->Acquire(); x = x + 1; mx->Release(); load add store load add store load add store load add store load add store load add store Holding a shared mutex prevents competing threads from entering a critical section. If the critical section code acquires the mutex, then its execution is serialized: only one thread runs it at a time. serialized atomic

81 How about this? x = x + 1; mx->Acquire(); x = x + 1; mx->Release(); load add store load add store A B

82 How about this? x = x + 1; mx->Acquire(); x = x + 1; mx->Release(); load add store load add store The locking discipline is not followed: purple fails to acquire the lock mx. Or rather: purple accesses the variable x through another program section A that is mutually critical with B, but does not acquire the mutex. A locking scheme is a convention that the entire program must follow. A B

83 How about this? mx->Acquire(); x = x + 1; mx->Release(); load add store load add store B lock->Acquire(); x = x + 1; lock->Release(); A

84 How about this? mx->Acquire(); x = x + 1; mx->Release(); load add store load add store This guy is not acquiring the right lock. Or whatever. Theyre not using the same lock, and thats what matters. A locking scheme is a convention that the entire program must follow. B lock->Acquire(); x = x + 1; lock->Release(); A

85 Using condition variables In typical use a condition variable is associated with some logical condition or predicate on the state protected by its mutex. – E.g., queue is empty, buffer is full, message in the mailbox. – Note: CVs are not variables. You can associate them with whatever data you want, i.e, the state protected by the mutex. A caller of CV wait must hold its mutex (be in the monitor). – This is crucial because it means that a waiter can wait on a logical condition and know that it wont change until the waiter is safely asleep. – Otherwise, another thread might change the condition and signal before the waiter is asleep! Signals do not stack! The waiter would sleep forever: the missed wakeup or wake-up waiter problem. The wait releases the mutex to sleep, and reacquires before return. – But another thread could have beaten the waiter to the mutex and messed with the condition: loop before you leap!

86 SharedLock: Reader/Writer Lock A reader/write lock or SharedLock is a new kind oflock that is similar to our old definition: – supports Acquire and Release primitives – assures mutual exclusion for writes to shared state But: a SharedLock provides better concurrency for readers when no writer is present. class SharedLock { AcquireRead(); /* shared mode */ AcquireWrite(); /* exclusive mode */ ReleaseRead(); ReleaseWrite(); }

87 Reader/Writer Lock Illustrated ArAr Multiple readers may hold the lock concurrently in shared mode. Writers always hold the lock in exclusive mode, and must wait for all readers or writer to exit. modereadwritemax allowed sharedyesnomany exclusive yesyesone not holdernonomany ArAr RrRr RrRr RwRw AwAw If each thread acquires the lock in exclusive (*write) mode, SharedLock functions exactly as an ordinary mutex.

88 Reader/Writer Lock: outline int i;/* # active readers, or -1 if writer */ void AcquireWrite() { while (i != 0) sleep….; i = -1; } void AcquireRead() { while (i < 0) sleep…; i += 1; } void ReleaseWrite() { i = 0; wakeup….; } void ReleaseRead() { i -= 1; if (i == 0) wakeup…; }

89 Reader/Writer Lock: adding a little mutex int i;/* # active readers, or -1 if writer */ Lock rwMx; AcquireWrite() { rwMx.Acquire(); while (i != 0) sleep…; i = -1; rwMx.Release(); } AcquireRead() { rwMx.Acquire(); while (i < 0) sleep…; i += 1; rwMx.Release(); } ReleaseWrite() { rwMx.Acquire(); i = 0; wakeup…; rwMx.Release(); } ReleaseRead() { rwMx.Acquire(); i -= 1; if (i == 0) wakeup…; rwMx.Release(); }

90 Reader/Writer Lock: cleaner syntax int i;/* # active readers, or -1 if writer */ Condition rwCv; /* bound to monitor mutex */ synchronized AcquireWrite() { while (i != 0) rwCv.Wait(); i = -1; } synchronized AcquireRead() { while (i < 0) rwCv.Wait(); i += 1; } synchronized ReleaseWrite() { i = 0; rwCv.Broadcast(); } synchronized ReleaseRead() { i -= 1; if (i == 0) rwCv.Signal(); } We can use Java syntax for convenience. Thats the beauty of pseudocode. We use any convenient syntax. These syntactic variants have the same meaning.

91 The Little Mutex Inside SharedLock ArAr ArAr RrRr RrRr RwRw ArAr AwAw RrRr

92 Limitations of the SharedLock Implementation This implementation has weaknesses discussed in [Birrell89]. – spurious lock conflicts (on a multiprocessor): multiple waiters contend for the mutex after a signal or broadcast. Solution: drop the mutex before signaling. (If the signal primitive permits it.) – spurious wakeups ReleaseWrite awakens writers as well as readers. Solution: add a separate condition variable for writers. – starvation How can we be sure that a waiting writer will ever pass its acquire if faced with a continuous stream of arriving readers?

93 Reader/Writer Lock: Second Try SharedLock::AcquireWrite() { rwMx.Acquire(); while (i != 0) wCv.Wait(&rwMx); i = -1; rwMx.Release(); } SharedLock::AcquireRead() { rwMx.Acquire(); while (i < 0)...rCv.Wait(&rwMx);... i += 1; rwMx.Release(); } SharedLock::ReleaseWrite() { rwMx.Acquire(); i = 0; if (readersWaiting) rCv.Broadcast(); else wCv.Signal(); rwMx.Release(); } SharedLock::ReleaseRead() { rwMx.Acquire(); i -= 1; if (i == 0) wCv.Signal(); rwMx.Release(); } Use two condition variables protected by the same mutex. We cant do this in Java, but we can still use Java syntax in our pseudocode. Be sure to declare the binding of CVs to mutexes!

94 Reader/Writer Lock: Second Try synchronized AcquireWrite() { while (i != 0) wCv.Wait(); i = -1; } synchronized AcquireRead() { while (i < 0) { readersWaiting+=1; rCv.Wait(); readersWaiting-=1; } i += 1; } synchronized ReleaseWrite() { i = 0; if (readersWaiting) rCv.Broadcast(); else wCv.Signal(); } synchronized ReleaseRead() { i -= 1; if (i == 0) wCv.Signal(); } wCv and rCv are protected by the monitor mutex.

95 Starvation The reader/writer lock example illustrates starvation: under load, a writer might be stalled forever by a stream of readers. Example: a one-lane bridge or tunnel. – Wait for oncoming car to exit the bridge before entering. – Repeat as necessary… Solution: some reader must politely stop before entering, even though it is not forced to wait by oncoming traffic. – More code… – More complexity…

96 Dining Philosophers N processes share N resources resource requests occur in pairs w/ random think times hungry philosopher grabs fork...and doesnt let go...until the other fork is free...and the linguine is eaten while(true) { Think(); AcquireForks(); Eat(); ReleaseForks(); } D B A C

97 Resource Graph or Wait-for Graph A vertex for each process and each resource If process A holds resource R, add an arc from R to A. 21 B A A grabs fork 1 B grabs fork 2

98 Resource Graph or Wait-for Graph A vertex for each process and each resource If process A holds resource R, add an arc from R to A. If process A is waiting for R, add an arc from A to R. 21 B A A grabs fork 1 and waits for fork 2. B grabs fork 2 and waits for fork 1.

99 Resource Graph or Wait-for Graph A vertex for each process and each resource If process A holds resource R, add an arc from R to A. If process A is waiting for R, add an arc from A to R. The system is deadlocked iff the wait-for graph has at least one cycle. 21 B A A grabs fork 1 and waits for fork 2. B grabs fork 2 and waits for fork 1.

100 Deadlock vs. starvation A deadlock is a situation in which a set of threads are all waiting for another thread to move. But none of the threads can move because they are all waiting for another thread to do it. Deadlocked threads sleep forever: the software freezes. It stops executing, stops taking input, stops generating output. There is no way out. Starvation (also called livelock) is different: some schedule exists that can exit the livelock state, and the scheduler may select it, even if the probability is low.

101 12 Y A1A2R2R1 A2 A1 R1 R2 RTG for Two Philosophers 12 X SnSn SmSm SnSn SmSm (There are really only 9 states we care about: the key transitions are acquire and release events.)

102 1 2 Y X A1A2R2R1 A2 A1 R1 R2 Two Philosophers Living Dangerously ???

103 12 Y X A1A2R2R1 A2 A1 R1 R2 The Inevitable Result This is a deadlock state: There are no legal transitions out of it.

104 Four Conditions for Deadlock Four conditions must be present for deadlock to occur: 1. Non-preemption of ownership. Resources are never taken away from the holder. 2. Exclusion. A resource has at most one holder. 3. Hold-and-wait. Holder blocks to wait for another resource to become available. 4. Circular waiting. Threads acquire resources in different orders.

105 Not All Schedules Lead to Collisions The scheduler+machine choose a schedule, i.e., a trajectory or path through the graph. – Synchronization constrains the schedule to avoid illegal states. – Some paths just happen to dodge dangerous states as well. What is the probability of deadlock? – How does the probability change as: think times increase? number of philosophers increases?

106 Dealing with Deadlock 1. Ignore it. Do you feel lucky? 2. Detect and recover. Check for cycles and break them by restarting activities (e.g., killing threads). 3. Prevent it. Break any precondition. – Keep it simple. Avoid blocking with any lock held. – Acquire nested locks in some predetermined order. – Acquire resources in advance of need; release all to retry. – Avoid surprise blocking at lower layers of your program. 4. Avoid it. – Deadlock can occur by allocating variable-size resource chunks from bounded pools: google Bankers algorithm.

107 Guidelines for Lock Granularity 1. Keep critical sections short. Push noncritical statements outside to reduce contention. 2. Limit lock overhead. Keep to a minimum the number of times mutexes are acquired and released. – Note tradeoff between contention and lock overhead. 3. Use as few mutexes as possible, but no fewer. – Choose lock scope carefully: if the operations on two different data structures can be separated, it may be more efficient to synchronize those structures with separate locks. – Add new locks only as needed to reduce contention.Correctness first, performance second!

108 More Locking Guidelines 1. Write code whose correctness is obvious. 2. Strive for symmetry. Show the Acquire/Release pairs. Factor locking out of interfaces. Acquire and Release at the same layer in your layer cake of abstractions and functions. 3. Hide locks behind interfaces. 4. Avoid nested locks. – If you must have them, try to impose a strict order. 5. Sleep high; lock low. – Where in the layer cake should you put your locks?

109 Guidelines for Condition Variables 1. Document the condition(s) associated with each CV. What are the waiters waiting for? When can a waiter expect a signal? 2. Recheck the condition after returning from a wait. Loop before you leap! Another thread may beat you to the mutex. The signaler may be careless. A single CV may have multiple conditions. 3. Dont forget: signals on CVs do not stack! A signal will be lost if nobody is waiting: always check the wait condition before calling wait.

110 Threads! Module A Module B T1T2 sleepwakeup deadlock! Module A Module B T1 T2 deadlock! callbacks calls [John Ousterhout 1995] Threads break abstraction.

111 Semaphore Now we introduce a new synchronization object type: semaphore. A semaphore is a hidden atomic integer counter with only increment (V) and decrement (P) operations. Decrement blocks iff the count is zero. Semaphores handle all of your synchronization needs with one elegant but confusing abstraction. V-Up P-Down int sem wait if (sem == 0) thenuntil a V

112 Example: binary semaphore A binary semaphore takes only values 0 and 1. It requires a usage constraint: the set of threads using the semaphore call P and V in strict alternation. – Never two V in a row. 1 0 P-Down V-Up wait P-Down wakeup on V

113 A mutex is a binary semaphore 1 0 P-Down V-Up wait P-Down wakeup on V V P PV Once a thread A completes its P, no other thread can P until A does a matching V. A mutex is just a binary semaphore with an initial value of 1, for which each thread calls P-V in strict pairs.

114 Semaphores vs. Condition Variables Semaphores are prefab CVs with an atomic integer. 1.V(Up) differs from signal (notify) in that: – Signal has no effect if no thread is waiting on the condition. Condition variables are not variables! They have no value! – Up has the same effect whether or not a thread is waiting. Semaphores retain a memory of calls to Up. 2. P(Down) differs from wait in that: – Down checks the condition and blocks only if necessary. No need to recheck the condition after returning from Down. The wait condition is defined internally, but is limited to a counter. – Wait is explicit: it does not check the condition itself, ever. Condition is defined externally and protected by integrated mutex.

115 Semaphore void P() { s = s - 1; } void V() { s = s + 1; } Step 0. Increment and decrement operations on a counter. But how to ensure that these operations are atomic, with mutual exclusion and no races? How to implement the blocking (sleep/wakeup) behavior of semaphores?

116 Semaphore void P() { synchronized(this) { …. s = s – 1; } void V() { synchronized(this) { s = s + 1; …. } Step 1. Use a mutex so that increment (V) and decrement (P) operations on the counter are atomic.

117 Semaphore synchronized void P() { s = s – 1; } synchronized void V() { s = s + 1; } Step 1. Use a mutex so that increment (V) and decrement (P) operations on the counter are atomic.

118 Semaphore synchronized void P() { while (s == 0) wait(); s = s - 1; } synchronized void V() { s = s + 1; if (s == 1) notify(); } Step 2. Use a condition variable to add sleep/wakeup synchronization around a zero count. (This is Java syntax.)

119 Semaphore synchronized void P() { if (s == 0) ………….; s = s - 1; ASSERT(s >= 0); } synchronized void V() { s = s + 1; ………. }

120 Semaphore synchronized void P() { while (s == 0) wait(); s = s - 1; ASSERT(s >= 0); } synchronized void V() { s = s + 1; signal(); } This code constitutes a proof that monitors (mutexes and condition variables) are at least as powerful as semaphores. Loop before you leap! Understand why the while is needed, and why an if is not good enough. Wait releases the monitor/mutex and blocks until a signal. Signal wakes up one waiter blocked in P, if there is one, else the signal has no effect: it is forgotten.

121 Fair? synchronized void P() { while (s == 0) wait(); s = s - 1; } synchronized void V() { s = s + 1; signal(); } Loop before you leap! But can a waiter be sure to eventually break out of this loop and consume a count? What if some other thread beats me to the lock (monitor) and completes a P before I wake up? V P VVVPPP Mesa semantics do not guarantee fairness.

122 Ping-pong with semaphores void PingPong() { while(not done) { blue->P(); Compute(); purple->V(); } void PingPong() { while(not done) { purple->P(); Compute(); blue->V(); } blue->Init(0); purple->Init(1);

123 Ping-pong with semaphores P V PV P 0101 PV V Compute The threads compute in strict alternation.

124 Ping-pong with semaphores void PingPong() { while(not done) { blue->P(); Compute(); purple->V(); } void PingPong() { while(not done) { purple->P(); Compute(); blue->V(); } blue->Init(0); purple->Init(1);

125 Basic barrier void Barrier() { while(not done) { blue->P(); Compute(); purple->V(); } void Barrier() { while(not done) { purple->P(); Compute(); blue->V(); } blue->Init(1); purple->Init(1);

126 Barrier with semaphores P V PV P V 1 P V Compute Neither thread can advance to the next iteration until its peer completes the current iteration. Compute

127 Basic producer/consumer void Produce(int m) { empty->P(); buf = m; full->V(); } int Consume() { int m; full->P(); m = buf; empty->V(); return(m); } empty->Init(1); full->Init(0); int buf; This use of a semaphore pair is called a split binary semaphore: the sum of the values is always one. Basic producer/consumer is called rendezvous: one producer, one consumer, and one item at a time. It is the same as ping-pong: producer and consumer access the buffer in strict alternation.

128 EXTRA SLIDES These are in scope, but were not discussed

129 Blocking kernel TCB active ready or running blocked wait sleep wait wakeup signal When a thread is blocked on a synchronization object (a mutex or CV) its TCB is placed on a sleep queue of threads waiting for an event on that object. This slide applies to the process abstraction too, or, more precisely, to the main thread of a process. How to synchronize thread queues and sleep/wakeup inside the kernel? Interrupts drive many wakeup events. sleep queueready queue

130 Managing threads: internals running readyblocked sleep STOP wait wakeup dispatch yield preempt sleep queueready queue A running thread may invoke an API of a synchronization object, and block. The code places the current threads TCB on a sleep queue, then initiates a context switch to another ready thread. If a thread is ready then its TCB is on a ready queue. Scheduler code running on an idle core may pick it up and context switch into the thread to run it. wakeup sleep dispatch running

131 Thread.Wakeup(SleepQueue q) { lock and disable; q.RemoveFromQ(this); this.status = READY; sched.AddToReadyQ(this); unlock and enable; } Thread.Sleep(SleepQueue q) { lock and disable interrupts; this.status = BLOCKED; q.AddToQ(this); next = sched.GetNextThreadToRun(); Switch(this, next); unlock and enable; } Sleep/wakeup: a rough idea This is pretty rough. Some issues to resolve: What if there are no ready threads? How does a thread terminate? How does the first thread start? Synchronization details vary.

132 What cores do ready queue (runqueue) scheduler getNextToRun() nothing? pause got thread sleep exit idle timer quantum expired run thread switch inswitch out Idle loop get thread put thread

133 Switching out What causes a core to switch out of the current thread? – Fault+sleep or fault+kill – Trap+sleep or trap+exit – Timer interrupt: quantum expired – Higher-priority thread becomes ready – …? run thread switch inswitch out Note: the thread switch-out cases are sleep, forced-yield, and exit, all of which occur in kernel mode following a trap, fault, or interrupt. But a trap, fault, or interrupt does not necessarily cause a thread switch!

134 Whats a race? Suppose we execute program P. The machine and scheduler choose a schedule S – S is a partial order of events. The events are loads and stores on shared memory locations, e.g., x. Suppose there is some x with a concurrent load and store to x. Then P has a race. A race is a bug. The behavior of P is not well-defined.

135 ArAr ArAr RrRr RrRr RwRw ArAr AwAw RrRr

136 Variations: looping producer Producer Infinite loop ok? Why/why not? Release lock in wait call producer () { lock (sodaLock) while (1) { while(numSodas==MaxSodas){ wait (sodaLock, hasRoom) } add soda to machine signal (hasSoda) } unlock (sodaLock) }

137 Variations: resting producer Producer Sleep ok? Why/why not? Shouldnt hold locks during a slow operation producer () { lock (sodaLock) while (1) { sleep (1 hour) while(numSodas==MaxSodas){ wait (sodaLock, hasRoom) } add soda to machine signal (hasSoda) } unlock (sodaLock) }

138 Variations: one CV? producer () { lock (sodaLock) while(numSodas==MaxSodas){ wait (sodaLock,hasRorS) } add one soda to machine signal(hasRorS) unlock (sodaLock) } consumer () { lock (sodaLock) while (numSodas == 0) { wait (sodaLock,hasRorS) } take a soda from machine signal (hasRorS) unlock (sodaLock) } Two producers, two consumers: who consumes a signal? ProducerA and ConsumerB wait while ConsumerC signals? MxCVMxCV

139 Variations: one CV? producer () { lock (sodaLock) while(numSodas==MaxSodas){ wait (sodaLock,hasRorS) } add one soda to machine signal (hasRorS) unlock (sodaLock) } consumer () { lock (sodaLock) while (numSodas == 0) { wait (sodaLock,hasRorS) } take a soda from machine signal (hasRorS) unlock (sodaLock) } Is it possible to have a producer and consumer both waiting? max=1, cA and cB wait, pC adds/signals, pD waits, cA wakes

140 Variations: one CV? producer () { lock (sodaLock) while(numSodas==MaxSodas){ wait (sodaLock,hasRorS) } add one soda to machine signal (hasRorS) unlock (sodaLock) } consumer () { lock (sodaLock) while (numSodas == 0) { wait (sodaLock,hasRorS) } take a soda from machine signal (hasRorS) unlock (sodaLock) } How can we make the one CV solution work?

141 Variations: one CV? producer () { lock (sodaLock) while(numSodas==MaxSodas){ wait (sodaLock,hasRorS) } add one soda to machine broadcast (hasRorS) unlock (sodaLock) } consumer () { lock (sodaLock) while (numSodas == 0) { wait (sodaLock,hasRorS) } take a soda from machine broadcast (hasRorS) unlock (sodaLock) } Use broadcast instead of signal: safe but slow.

142 Broadcast vs signal Can I always use broadcast instead of signal? Yes, assuming threads recheck condition And they should: loop before you leap! Mesa semantics requires it anyway: another thread could get to the lock before wait returns. Why might I use signal instead? Efficiency (spurious wakeups) May wakeup threads for no good reason Signal is just a performance hint. lpcox slide edited by chase

143 Example: the soda/HFCS machine Vending machine (buffer) Soda drinker (consumer) Delivery person (producer)

144 Prod.-cons. with semaphores Same before-after constraints If buffer empty, consumer waits for producer If buffer full, producer waits for consumer Semaphore assignments mutex (binary semaphore) fullBuffers (counts number of full slots) emptyBuffers (counts number of empty slots)

145 Prod.-cons. with semaphores Initial semaphore values? Mutual exclusion sem mutex (?) Machine is initially empty sem fullBuffers (?) sem emptyBuffers (?)

146 Prod.-cons. with semaphores Initial semaphore values Mutual exclusion sem mutex (1) Machine is initially empty sem fullBuffers (0) sem emptyBuffers (MaxSodas)

147 Prod.-cons. with semaphores producer () { one less empty buffer down (emptyBuffers) put one soda in one more full buffer up (fullBuffers) } consumer () { one less full buffer down (fullBuffers) take one soda out one more empty buffer up (emptyBuffers) } Semaphore fullBuffers(0),emptyBuffers(MaxSodas) Semaphores give us elegant full/empty synchronization. Is that enough?

148 Prod.-cons. with semaphores producer () { down (emptyBuffers) down (mutex) put one soda in up (mutex) up (fullBuffers) } consumer () { down (fullBuffers) down (mutex) take one soda out up (mutex) up (emptyBuffers) } Semaphore mutex(1),fullBuffers(0),emptyBuffers(MaxSodas) Use one semaphore for fullBuffers and emptyBuffers?

149 Prod.-cons. with semaphores Does the order of the down calls matter? Yes. Can cause deadlock. producer () { down (mutex) down (emptyBuffers) put soda in up (fullBuffers) up (mutex) } consumer () { down (mutex) down (fullBuffers) take soda out up (emptyBuffers) up (mutex) } Semaphore mutex(1),fullBuffers(0),emptyBuffers(MaxSodas) 2 1

150 Prod.-cons. with semaphores Does the order of the up calls matter? Not for correctness (possible efficiency issues). producer () { down (emptyBuffers) down (mutex) put soda in up (fullBuffers) up (mutex) } consumer () { down (fullBuffers) down (mutex) take soda out up (emptyBuffers) up (mutex) } Semaphore mutex(1),fullBuffers(0),emptyBuffers(MaxSodas)

151 Prod.-cons. with semaphores What about multiple consumers and/or producers? Doesnt matter; solution stands. producer () { down (emptyBuffers) down (mutex) put soda in up (mutex) up (fullBuffers) } consumer () { down (fullBuffers) down (mutex) take soda out up (mutex) up (emptyBuffers) } Semaphore mutex(1),fullBuffers(0),emptyBuffers(MaxSodas)

152 Prod.-cons. with semaphores What if 1 full buffer and multiple consumers call down? Only one will see semaphore at 1, rest see at 0. producer () { down (emptyBuffers) down (mutex) put soda in up (mutex) up (fullBuffers) } consumer () { down (fullBuffers) down (mutex) take soda out up (mutex) up (emptyBuffers) } Semaphore mtx(1),fullBuffers(1),emptyBuffers(MaxSodas-1)

153 Monitors vs. semaphores Monitors Separate mutual exclusion and wait/signal Semaphores Provide both with same mechanism Semaphores are more elegant At least for producer/consumer Can be harder to program

154 Monitors vs. semaphores Where are the conditions in both? Which is more flexible? Why do monitors need a lock, but not semaphores? // Monitors lock (mutex) while (condition) { wait (CV, mutex) } unlock (mutex) // Semaphores down (semaphore)

155 Monitors vs. semaphores When are semaphores appropriate? When shared integer maps naturally to problem at hand (i.e. when the condition involves a count of one thing) // Monitors lock (mutex) while (condition) { wait (CV, mutex) } unlock (mutex) // Semaphores down (semaphore)

156 Reader/Writer with Semaphores SharedLock::AcquireRead() { rmx.P(); if (first reader) wsem.P(); rmx.V(); } SharedLock::ReleaseRead() { rmx.P(); if (last reader) wsem.V(); rmx.V(); } SharedLock::AcquireWrite() { wsem.P(); } SharedLock::ReleaseWrite() { wsem.V(); }

157 SharedLock with Semaphores: Take 2 (outline) SharedLock::AcquireRead() { rblock.P(); if (first reader) wsem.P(); rblock.V(); } SharedLock::ReleaseRead() { if (last reader) wsem.V(); } SharedLock::AcquireWrite() { if (first writer) rblock.P(); wsem.P(); } SharedLock::ReleaseWrite() { wsem.V(); if (last writer) rblock.V(); } The rblock prevents readers from entering while writers are waiting. Note: the marked critical systems must be locked down with mutexes. Note also: semaphore wakeup chain replaces broadcast or notifyAll.

158 SharedLock with Semaphores: Take 2 SharedLock::AcquireRead() { rblock.P(); rmx.P(); if (first reader) wsem.P(); rmx.V(); rblock.V(); } SharedLock::ReleaseRead() { rmx.P(); if (last reader) wsem.V(); rmx.V(); } SharedLock::AcquireWrite() { wmx.P(); if (first writer) rblock.P(); wmx.V(); wsem.P(); } SharedLock::ReleaseWrite() { wsem.V(); wmx.P(); if (last writer) rblock.V(); wmx.V(); } Added for completeness.

159 EventBarrier arrive() complete() raise() controller eb.arrive(); crossBridge(); eb.complete(); …. eb.raise(); …

160

161 EXTRA SLIDES These are in NOT scope, and were not discussed, but may help improve your understanding.

162 Wakeup from interrupt handler sleep ready queue interrupt trap or fault return to user mode wakeup sleep queue switch Examples? Note: interrupt handlers do not block: typically there is a single interrupt stack for each core that can take interrupts. If an interrupt arrived while another handler was sleeping, it would corrupt the interrupt stack.

163 Wakeup from interrupt handler sleep ready queue interrupt trap or fault return to user mode wakeup sleep queue switch How should an interrupt handler wakeup a thread? Condition variable signal? Semaphore V?

164 Interrupts An arriving interrupt transfers control immediately to the corresponding handler (Interrupt Service Routine). ISR runs kernel code in kernel mode in kernel space. Interrupts may be nested according to priority. executing thread low-priority handler (ISR) high-priority ISR

165 Interrupt priority: rough sketch N interrupt priority classes When an ISR at priority p runs, CPU blocks interrupts of priority p or lower. Kernel software can query/raise/lower the CPU interrupt priority level (IPL). – Defer or mask delivery of interrupts at that IPL or lower. – Avoid races with higher-priority ISR by raising CPU IPL to that priority. – e.g., BSD Unix spl*/splx primitives. Summary: Kernel code can enable/disable interrupts as needed. splx(s) clock splimp splbio splnet spl0 low high BSD example int s; s = splhigh(); /* all interrupts disabled */ splx(s); /* IPL is restored to s */

166 What ISRs do Interrupt handlers: – bump counters, set flags – throw packets on queues – … – wakeup waiting threads Wakeup puts a thread on the ready queue. Use spinlocks for the queues But how do we synchronize with interrupt handlers?

167 Spinlocks in the kernel We have basic mutual exclusion that is very useful inside the kernel, e.g., for access to thread queues. – Spinlocks based on atomic instructions. – Can synchronize access to sleep/ready queues used to implement higher-level synchronization objects. Dont use spinlocks from user space! A thread holding a spinlock could be preempted at any time. – If a thread is preempted while holding a spinlock, then other threads/cores may waste many cycles spinning on the lock. – Thats a kernel/thread library integration issue: fast spinlock synchronization in user space is a research topic. But spinlocks are very useful in the kernel, esp. for synchronizing with interrupt handlers!

168 Synchronizing with ISRs Interrupt delivery can cause a race if the ISR shares data (e.g., a thread queue) with the interrupted code. Example: Core at IPL=0 (thread context) holds spinlock, interrupt is raised, ISR attempts to acquire spinlock…. That would be bad. Disable interrupts. executing thread (IPL 0) in kernel mode disable interrupts for critical section int s; s = splhigh(); /* critical section */ splx(s);

169 Obviously this is just example detail from a particular machine (IA32): the details arent important.

170 Memory ordering Shared memory is complex on multicore systems. Does a load from a memory location (address) return the latest value written to that memory location by a store? What does latest mean in a parallel system? T1 M W(x)=1 W(y)=1 OK R(y) 1 T2 It is common to presume that load and store ops execute sequentially on a shared memory, and a store is immediately and simultaneously visible to load at all other threads. But not on real machines. R(x) 1

171 Memory ordering A load might fetch from the local cache and not from memory. A store may buffer a value in a local cache before draining the value to memory, where other cores can access it. Therefore, a load from one core does not necessarily return the latest value written by a store from another core. T1 M W(x)=1 W(y)=1 OK R(y) 0?? T2 A trick called Dekkers algorithm supports mutual exclusion on multi-core without using atomic instructions. It assumes that load and store ops on a given location execute sequentially. But they dont. R(x) 0??

172 The first thing to understand about memory behavior on multi-core systems Cores must see a consistent view of shared memory for programs to work properly. But what does it mean? Synchronization accesses tell the machine that ordering matters: a happens-before relationship exists. Machines always respect that. – Modern machines work for race-free programs. – Otherwise, all bets are off. Synchronize! T1 M W(x)=1 W(y)=1 OK R(y) 1 T2 The most you should assume is that any memory store before a lock release is visible to a load on a core that has subsequently acquired the same lock. R(x) 0?? pass lock

173 A peek at some deep tech mx->Acquire(); x = x + 1; mx->Release(); mx->Acquire(); x = x + 1; mx->Release(); happens before (<) before An execution schedule defines a partial order of program events. The ordering relation (<) is called happens-before. 1.Events within a thread are ordered. 2.Mutex handoff orders events across threads: the release #N happens- before acquire #N+1. 3.Happens-before is transitive: if (A < B) and (B < C) then A < C. Two events are concurrent if neither happens-before the other. They might execute in some order, but only by luck. Just three rules govern happens-before order: The next schedule may reorder them. Machines may reorder concurrent events, but they always respect happens-before ordering.

174 The point of all that We use special atomic instructions to implement locks. E.g., a TSL or CMPXCHG on a lock variable lockvar is a synchronization access. Synchronization accesses also have special behavior with respect to the memory system. – Suppose core C1 executes a synchronization access to lockvar at time t1, and then core C2 executes a synchronization access to lockvar at time t2. – Then t1

175 7.1. LOCKED ATOMIC OPERATIONS The 32-bit IA-32 processors support locked atomic operations on locations in system memory. These operations are typically used to manage shared data structures (such as semaphores, segment descriptors, system segments, or page tables) in which two or more processors may try simultaneously to modify the same field or flag…. Note that the mechanisms for handling locked atomic operations have evolved as the complexity of IA-32 processors has evolved…. Synchronization mechanisms in multiple-processor systems may depend upon a strong memory-ordering model. Here, a program can use a locking instruction such as the XCHG instruction or the LOCK prefix to insure that a read-modify-write operation on memory is carried out atomically. Locking operations typically operate like I/O operations in that they wait for all previous instructions to complete and for all buffered writes to drain to memory…. This is just an example of a principle on a particular machine (IA32): these details arent important.

176 Example: Unix Sleep (BSD) sleep (void* event, int sleep_priority) { struct proc *p = curproc; int s; s = splhigh(); /* disable all interrupts */ p->p_wchan = event; /* what are we waiting for */ p->p_priority -> priority; /* wakeup scheduler priority */ p->p_stat = SSLEEP; /* transition curproc to sleep state */ INSERTQ(&slpque[HASH(event)], p);/* fiddle sleep queue */ splx(s); /* enable interrupts */ mi_switch(); /* context switch */ /* were back... */ } Illustration Only

177 Thread context switch 0 high code library data registers CPU (core) R0 Rn PC x x program common runtime stack address space SP y y stack 1. save registers 2. load registers switch in switch out

178 /* * Save context of the calling thread (old), restore registers of * the next thread to run (new), and return in context of new. */ switch/MIPS (old, new) { old->stackTop = SP; save RA in old->MachineState[PC]; save callee registers in old->MachineState restore callee registers from new->MachineState RA = new->MachineState[PC]; SP = new->stackTop; return (to RA) } This example (from the old MIPS ISA) illustrates how context switch saves/restores the user register context for a thread, efficiently and without assigning a value directly into the PC.

179 switch/MIPS (old, new) { old->stackTop = SP; save RA in old->MachineState[PC]; save callee registers in old->MachineState restore callee registers from new->MachineState RA = new->MachineState[PC]; SP = new->stackTop; return (to RA) } Example: Switch() Caller-saved registers (if needed) are already saved on its stack, and restored automatically on return. Return to procedure that called switch in new thread. Save current stack pointer and callers return address in old thread object. Switch off of old stack and over to new stack. RA is the return address register. It contains the address that a procedure return instruction branches to.

180 What to know about context switch The Switch/MIPS example is an illustration for those of you who are interested. It is not required to study it. But you should understand how a thread system would use it (refer to state transition diagram): Switch() is a procedure that returns immediately, but it returns onto the stack of new thread, and not in the old thread that called it. Switch() is called from internal routines to sleep or yield (or exit). Therefore, every thread in the blocked or ready state has a frame for Switch() on top of its stack: it was the last frame pushed on the stack before the thread switched out. (Need per-thread stacks to block.) The thread create primitive seeds a Switch() frame manually on the stack of the new thread, since it is too young to have switched before. When a thread switches into the running state, it always returns immediately from Switch() back to the internal sleep or yield routine, and from there back on its way to wherever it goes next.

181 Recap: threads on the metal An OS implements synchronization objects using a combination of elements: – Basic sleep/wakeup primitives of some form. – Sleep places the thread TCB on a sleep queue and does a context switch to the next ready thread. – Wakeup places each awakened thread on a ready queue, from which the ready thread is dispatched to a core. – Synchronization for the thread queues uses spinlocks based on atomic instructions, together with interrupt enable/disable. – The low-level details are tricky and machine-dependent. – The atomic instructions (synchronization accesses) also drive memory consistency behaviors in the machine, e.g., a safe memory model for fully synchronized race-free programs. – Watch out for interrupts! Disable/enable as needed.

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