CS162 Operating Systems and Systems Programming Lecture 4 Synchronization, Atomic operations, Locks, SemaphoresSpace Shuttle ExampleAnother Concurrent Program ExampleGoals for TodayMotivation: “Too much milk”DefinitionsMore DefinitionsToo Much Milk: Correctness PropertiesToo Much Milk: Solution #1Slide 10Too Much Milk: Solution #1½Too Much Milk Solution #2Slide 13Too Much Milk Solution #2: problem!Review: Too Much Milk Solution #3Review: Solution #3 discussionHigh-Level PictureToo Much Milk: Solution #4How to implement Locks?Naïve use of Interrupt Enable/DisableBetter Implementation of Locks by Disabling InterruptsProject SignupSlide 235min BreakNew Lock Implementation: DiscussionInterrupt re-enable in going to sleepHow to Re-enable After Sleep()?Atomic Read-Modify-Write instructionsExamples of Read-Modify-WriteImplementing Locks with test&setProblem: Busy-Waiting for LockBetter Locks using test&setSlide 33Higher-level Primitives than LocksSemaphoresSemaphores Like Integers ExceptTwo Uses of SemaphoresSummaryCS162Operating Systems andSystems ProgrammingLecture 4Synchronization, Atomic operations, Locks, SemaphoresJanuary 31, 2011Ion Stoicahttp://inst.eecs.berkeley.edu/~cs162Lec 4.231/1/11 Ion Stoica CS162 ©UCB Spring 2011Space Shuttle Example•Original Space Shuttle launch aborted 20 minutes before scheduled launch•Shuttle has five computers:–Four run the “Primary Avionics Software System” (PASS)»Asynchronous and real-time»Runs all of the control systems»Results synchronized and compared every 3 to 4 ms–The Fifth computer is the “Backup Flight System” (BFS)»stays synchronized in case it is needed»Written by completely different team than PASS•Countdown aborted because BFS disagreed with PASS–A 1/67 chance that PASS was out of sync one cycle–Bug due to modifications in initialization code of PASS»A delayed init request placed into timer queue»As a result, timer queue not empty at expected time to force use of hardware clock–Bug not found during extensive simulationPASSBFSLec 4.331/1/11 Ion Stoica CS162 ©UCB Spring 2011Another Concurrent Program Example•Two threads, A and B, compete with each other–One tries to increment a shared counter–The other tries to decrement the counterThread A Thread Bi = 0; i = 0;while (i < 10) while (i > -10) i = i + 1; i = i – 1;printf(“A wins!”); printf(“B wins!”);•Assume that memory loads and stores are atomic, but incrementing and decrementing are not atomic •Who wins? •Is it guaranteed that someone wins? Why or why not?•What it both threads have their own CPU running at same speed? Is it guaranteed that it goes on forever?Lec 4.431/1/11 Ion Stoica CS162 ©UCB Spring 2011Goals for Today•Synchronization•Hardware Support for Synchronization•Higher-level Synchronization Abstractions–Semaphores, monitors, and condition variables•Programming paradigms for concurrent programsNote: Some slides and/or pictures in the following areadapted from slides ©2005 Silberschatz, Galvin, and Gagne Note: Some slides and/or pictures in the following areadapted from slides ©2005 Silberschatz, Galvin, and Gagne. Many slides generated by Kubiatowicz.Lec 4.531/1/11 Ion Stoica CS162 ©UCB Spring 2011Motivation: “Too much milk”•Great thing about OS’s – analogy between problems in OS and problems in real life–Help you understand real life problems better–But, computers are much stupider than people•Example: People need to coordinate:Arrive home, put milk away3:30Buy milk3:25Arrive at storeArrive home, put milk away3:20Leave for storeBuy milk3:15Leave for store3:05Look in Fridge. Out of milk3:00Look in Fridge. Out of milkArrive at store3:10Person BPerson ATimeLec 4.631/1/11 Ion Stoica CS162 ©UCB Spring 2011Definitions•Synchronization: using atomic operations to ensure cooperation between threads–For now, only loads and stores are atomic–We’ll show its hard to build anything useful with only reads and writes•Mutual Exclusion: ensuring that only one thread does a particular thing at a time–One thread excludes the other while doing its task•Critical Section: piece of code that only one thread can execute at once–Critical section is the result of mutual exclusion–Critical section and mutual exclusion are two ways of describing the same thing.Lec 4.731/1/11 Ion Stoica CS162 ©UCB Spring 2011More Definitions•Lock: prevents someone from doing something–Lock before entering critical section and before accessing shared data–Unlock when leaving, after accessing shared data–Wait if locked»Important idea: all synchronization involves waiting•For example: fix the milk problem by putting a key on the refrigerator–Lock it and take key if you are going to go buy milk–Fixes too much: roommate angry if only wants orange juice–Of Course – We don’t know how to make a lock yet#$@%@#$@Lec 4.831/1/11 Ion Stoica CS162 ©UCB Spring 2011Too Much Milk: Correctness Properties•Need to be careful about correctness of concurrent programs, since non-deterministic–Always write down behavior first–Impulse is to start coding first, then when it doesn’t work, pull hair out–Instead, think first, then code•What are the correctness properties for the “Too much milk” problem?–Never more than one person buys–Someone buys if needed•Restrict ourselves to use only atomic load and store operations as building blocksLec 4.931/1/11 Ion Stoica CS162 ©UCB Spring 2011Too Much Milk: Solution #1•Use a note to avoid buying too much milk:–Leave a note before buying (kind of “lock”)–Remove note after buying (kind of “unlock”)–Don’t buy if note (wait)•Suppose a computer tries this (remember, only memory read/write are atomic):if (noMilk) { if (noNote) { leave Note; buy milk; remove note; }}•Result?Lec 4.1031/1/11 Ion Stoica CS162 ©UCB Spring 2011Too Much Milk: Solution #1•Still too much milk but only occasionally! Thread A Thread B if (noMilk) if (noNote) { if (noMilk) if (noNote) { leave Note;buy milk; remove note; } } leave Note; buy milk; …•Thread can get context switched after checking milk and note but before buying milk!•Solution makes problem worse since fails intermittently–Makes it really hard to debug…–Must work despite what the thread dispatcher does!Lec 4.1131/1/11 Ion Stoica CS162 ©UCB Spring 2011Too Much Milk: Solution #1½ •Clearly the Note
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