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<a class="dropdown-item" tabindex="-1" href="SynchOverview.html#"><b>Chapter 1</b></a>
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<a class="dropdown-item" href="IntroConcSysOverview.html"> 1.1. Introduction to Concurrent Systems</a>
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<a class="dropdown-item" href="SysAndModels.html"> 1.2. Systems and Models</a>
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<a class="dropdown-item disabled"><b>Chapter 2</b></a>
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<a class="dropdown-item" href="ProcessesOverview.html"> 2.1. Processes and OS Basics</a>
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<a class="dropdown-item" href="Multiprogramming.html"> 2.2. Processes and Multiprogramming</a>
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<a class="dropdown-item" href="KernelMechanics.html"> 2.3. Kernel Mechanics</a>
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<a class="dropdown-item" href="Syscall.html"> 2.4. System Call Interface</a>
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<a class="dropdown-item" href="ProcessCycle.html"> 2.5. Process Life Cycle</a>
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<a class="dropdown-item" href="UnixFile.html"> 2.6. The UNIX File Abstraction</a>
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<a class="dropdown-item" href="EventsSignals.html"> 2.7. Events and Signals</a>
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<a class="dropdown-item" href="Extended2Processes.html"> 2.8. Extended Example: Listing Files with Processes</a>
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<a class="dropdown-item disabled"><b>Chapter 3</b></a>
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<a class="dropdown-item" href="IPCOverview.html"> 3.1. Concurrency with IPC</a>
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<a class="dropdown-item" href="IPCModels.html"> 3.2. IPC Models</a>
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<a class="dropdown-item" href="Pipes.html"> 3.3. Pipes and FIFOs</a>
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<a class="dropdown-item" href="MMap.html"> 3.4. Shared Memory With Memory-mapped Files</a>
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<a class="dropdown-item" href="POSIXvSysV.html"> 3.5. POSIX vs. System V IPC</a>
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<a class="dropdown-item" href="MQueues.html"> 3.6. Message Passing With Message Queues</a>
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<a class="dropdown-item" href="ShMem.html"> 3.7. Shared Memory</a>
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<a class="dropdown-item" href="IPCSems.html"> 3.8. Semaphores</a>
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<a class="dropdown-item" href="Extended3Bash.html"> 3.9. Extended Example: Bash-lite: A Simple Command-line Shell</a>
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<a class="dropdown-item disabled"><b>Chapter 4</b></a>
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<a class="dropdown-item" href="SocketsOverview.html"> 4.1. Networked Concurrency</a>
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<a class="dropdown-item" href="FiveLayer.html"> 4.2. The TCP/IP Internet Model</a>
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<a class="dropdown-item" href="NetApps.html"> 4.3. Network Applications and Protocols</a>
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<a class="dropdown-item" href="Sockets.html"> 4.4. The Socket Interface</a>
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<a class="dropdown-item" href="TCPSockets.html"> 4.5. TCP Socket Programming: HTTP</a>
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<a class="dropdown-item" href="UDPSockets.html"> 4.6. UDP Socket Programming: DNS</a>
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<a class="dropdown-item" href="AppBroadcast.html"> 4.7. Application-Layer Broadcasting: DHCP</a>
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<a class="dropdown-item" href="Extended4CGI.html"> 4.8. Extended Example: CGI Web Server</a>
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<a class="dropdown-item disabled"><b>Chapter 5</b></a>
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<a class="dropdown-item" href="InternetOverview.html"> 5.1. The Internet and Connectivity</a>
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<a class="dropdown-item" href="Wireless.html"> 5.7. Wireless Connectivity: Wi-Fi, Bluetooth, and Zigbee</a>
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<a class="dropdown-item disabled"><b>Chapter 6</b></a>
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<a class="dropdown-item" href="ThreadsOverview.html"> 6.1. Concurrency with Multithreading</a>
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<a class="dropdown-item" href="RaceConditions.html"> 6.3. Race Conditions and Critical Sections</a>
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<a class="dropdown-item" href="ThreadArgs.html"> 6.5. Thread Arguments and Return Values</a>
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<a class="dropdown-item" href="ImplicitThreads.html"> 6.6. Implicit Threading and Language-based Threads</a>
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<a class="dropdown-item" href="Extended6Input.html"> 6.7. Extended Example: Keyboard Input Listener</a>
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<a class="dropdown-item" href="Extended6Primes.html"> 6.8. Extended Example: Concurrent Prime Number Search</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 7</b></a>
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<a class="dropdown-item" href="SynchOverview.html"> 7.1. Synchronization Primitives</a>
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<a class="dropdown-item" href="CritSect.html"> 7.2. Critical Sections and Peterson's Solution</a>
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<a class="dropdown-item" href="Locks.html"> 7.3. Locks</a>
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<a class="dropdown-item" href="Semaphores.html"> 7.4. Semaphores</a>
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<a class="dropdown-item" href="Barriers.html"> 7.5. Barriers</a>
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<a class="dropdown-item" href="Condvars.html"> 7.6. Condition Variables</a>
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<a class="dropdown-item" href="Deadlock.html"> 7.7. Deadlock</a>
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<a class="dropdown-item" href="Extended7Events.html"> 7.8. Extended Example: Event Log File</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 8</b></a>
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<a class="dropdown-item" href="SynchProblemsOverview.html"> 8.1. Synchronization Patterns and Problems</a>
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<a class="dropdown-item" href="SynchDesign.html"> 8.2. Basic Synchronization Design Patterns</a>
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<a class="dropdown-item" href="ProdCons.html"> 8.3. Producer-Consumer Problem</a>
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<a class="dropdown-item" href="ReadWrite.html"> 8.4. Readers-Writers Problem</a>
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<a class="dropdown-item" href="DiningPhil.html"> 8.5. Dining Philosophers Problem and Deadlock</a>
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<a class="dropdown-item" href="CigSmokers.html"> 8.6. Cigarette Smokers Problem and the Limits of Semaphores and Locks</a>
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<a class="dropdown-item" href="Extended8ModExp.html"> 8.7. Extended Example: Parallel Modular Exponentiation</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 9</b></a>
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<a class="dropdown-item" href="ParallelDistributedOverview.html"> 9.1. Parallel and Distributed Systems</a>
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<a class="dropdown-item" href="ParVConc.html"> 9.2. Parallelism vs. Concurrency</a>
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<a class="dropdown-item" href="ParallelDesign.html"> 9.3. Parallel Design Patterns</a>
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<a class="dropdown-item" href="Scaling.html"> 9.4. Limits of Parallelism and Scaling</a>
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<a class="dropdown-item" href="DistTiming.html"> 9.5. Timing in Distributed Environments</a>
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<a class="dropdown-item" href="DistDataStorage.html"> 9.6. Reliable Data Storage and Location</a>
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<a class="dropdown-item" href="DistConsensus.html"> 9.7. Consensus in Distributed Systems</a>
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<a class="dropdown-item" href="Extended9Blockchain.html"> 9.8. Extended Example: Blockchain Proof-of-Work</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Appendix A</b></a>
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<a class="dropdown-item" href="CLangOverview.html"> A.1. C Language Reintroduction</a>
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<a class="dropdown-item" href="Debugging.html"> A.2. Documentation and Debugging</a>
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<a class="dropdown-item" href="BasicTypes.html"> A.3. Basic Types and Pointers</a>
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<a class="dropdown-item" href="Arrays.html"> A.4. Arrays, Structs, Enums, and Type Definitions</a>
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<a class="dropdown-item" href="Functions.html"> A.5. Functions and Scope</a>
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<a class="dropdown-item" href="Pointers.html"> A.6. Pointers and Dynamic Allocation</a>
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<a class="dropdown-item" href="Strings.html"> A.7. Strings</a>
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<a class="dropdown-item" href="FunctionPointers.html"> A.8. Function Pointers</a>
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<a class="dropdown-item" href="Files.html"> A.9. Files</a>
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</div>
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</li>
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«  <a id="prevmod" href="Extended6Primes.html">6.8. Extended Example: Concurrent Prime Number Search</a>
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  ::  
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<a class="uplink" href="index.html">Contents</a>
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  ::  
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<a id="nextmod" href="CritSect.html">7.2. Critical Sections and Peterson’s Solution</a>  »
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<script>ODSA.SETTINGS.DISP_MOD_COMP = true;ODSA.SETTINGS.MODULE_NAME = "SynchOverview";ODSA.SETTINGS.MODULE_LONG_NAME = "Synchronization Primitives";ODSA.SETTINGS.MODULE_CHAPTER = "Synchronization Primitives"; ODSA.SETTINGS.BUILD_DATE = "2021-06-01 15:31:50"; ODSA.SETTINGS.BUILD_CMAP = false;JSAV_OPTIONS['lang']='en';JSAV_EXERCISE_OPTIONS['code']='java';</script><div class="section" id="synchronization-primitives">
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<h1>7.1. Synchronization Primitives<a class="headerlink" href="SynchOverview.html#synchronization-primitives" title="Permalink to this headline">¶</a></h1>
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<div class="figure mb-2 align-center" style="width: 100%">
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<a class="reference internal image-reference" href="_images/CSF-Timeline.7.png"><img class="p-3 mb-2 align-center border border-dark rounded-lg" alt="Timeline of major CSF topics with Multicore and Threads highlighted" src="_images/CSF-Timeline.7.png" style="width: 90%;" /></a>
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</div>
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<blockquote class="blockquote text-center">
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<p>“It’s not an idea until you write it down.”
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<footer class="blockquote-footer">Ivan Sutherland</footer>
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</blockquote><p>The resurgence of interest in multithreading in the late 1990s brought
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synchronization with it as a core topic. When multiple threads share an address
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space, they must ensure not to conflict in their memory accesses. Edsger
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Dijkstra’s THE OS introduced semaphores as a software construct in 1968, while
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the POSIX.1c-1995 standard defined an interface for them. In addition,
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higher-level primitives have made synchronization more manageable as a key
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infrastructure component for concurrent software.</p>
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<div class="topic border border-dark rounded-lg bg-light px-2 mb-3">
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<div class="figure align-left" style="width: 5%">
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<a class="reference internal image-reference" href="_images/CSF-Images-Objectives.png"><img alt="Decorative chapter objectives image" src="_images/CSF-Images-Objectives.png" style="width: 100%;" /></a>
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</div>
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<p class="topic-title first pt-2 mb-1">Chapter Objectives</p><hr class="mt-1" />
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<p>In this chapter, we will address the following instructional objectives:</p>
|
|||
|
|
<ul class="simple">
|
|||
|
|
<li>We will justify the need for synchronization primitives as solutions to race
|
|||
|
|
conditions and timing constraints.</li>
|
|||
|
|
<li>We will compare and contrast primitive synchronization mechanisms and what
|
|||
|
|
problems they solve.</li>
|
|||
|
|
<li>We will examine code using the POSIX thread library’s synchronization
|
|||
|
|
primitive implementations.</li>
|
|||
|
|
<li>We will identify the conditions that lead to deadlock, a difficult race
|
|||
|
|
condition that can arise in synchronized software.</li>
|
|||
|
|
</ul>
|
|||
|
|
</div>
|
|||
|
|
<p>Concurrent programs introduce a new class of software bugs called <a class="reference internal" href="Glossary.html#term-race-condition"><span class="xref std std-term">race
|
|||
|
|
conditions</span></a>. Race conditions arise whenever the <em>timing</em> of
|
|||
|
|
multiple <a class="reference internal" href="Glossary.html#term-thread"><span class="xref std std-term">threads of execution</span></a> determines the outcome. The
|
|||
|
|
problem is that the scheduling of threads is out of the control of the
|
|||
|
|
programmer who wrote the code and the user running the program.</p>
|
|||
|
|
<p>The scheduling is determined by the operating system kernel, a special program
|
|||
|
|
that manages the system’s resources. In modern general-purpose computer systems,
|
|||
|
|
there may be hundreds or thousands of threads that must take turns getting
|
|||
|
|
access to a small number of CPU processing cores. The kernel makes the decision
|
|||
|
|
of which thread to run based on what else the machine is doing, how long each
|
|||
|
|
thread has already run, and many other factors. The programmer cannot predict or
|
|||
|
|
control any of these factors; in fact, the exact combination of factors is
|
|||
|
|
likely to be unique every time the program runs.</p>
|
|||
|
|
<p>In many cases, the <a class="reference internal" href="Glossary.html#term-nondeterminism"><span class="xref std std-term">nondeterministic</span></a> nature of thread scheduling is not a
|
|||
|
|
problem. If one thread is responsible for factoring 5,182,397,724,980 into a
|
|||
|
|
product of primes while another is computing the billionth digit of Pi, it
|
|||
|
|
probably doesn’t matter which calculation finishes first. However, if these two
|
|||
|
|
results are going to be combined in some way, the program must guarantee that
|
|||
|
|
both of them have completed before attempting this third step. That is, these
|
|||
|
|
steps must be <a class="reference internal" href="Glossary.html#term-synchronization"><span class="xref std std-term">synchronized</span></a>.</p>
|
|||
|
|
<p>There are several forms of synchronization that various programs may require.
|
|||
|
|
Some common examples include the following:</p>
|
|||
|
|
<blockquote>
|
|||
|
|
<div><ul class="simple">
|
|||
|
|
<li>Multiple threads may try to modify a shared data structure, but each write
|
|||
|
|
must be performed one at a time to ensure the final result is correct.</li>
|
|||
|
|
<li>The system may need to place a limit on the number of simultaneous accesses to
|
|||
|
|
a shared resource to avoid delays. For instance, a web server may limit the
|
|||
|
|
number of incoming network requests to make sure it does not consume too much memory.</li>
|
|||
|
|
<li>Particular events may need to be performed in a specific order, such as when
|
|||
|
|
the output produced by one thread is required as input to another thread.</li>
|
|||
|
|
<li>The system may need to make sure that a minimum number of calculations have
|
|||
|
|
been completed before proceeding to take some sort of action. For example, some
|
|||
|
|
programs run multiple different algorithms that should produce the same result;
|
|||
|
|
the system may require at least three algorithms agree before having the
|
|||
|
|
confidence that the result is correct.</li>
|
|||
|
|
</ul>
|
|||
|
|
</div></blockquote>
|
|||
|
|
<p>The <a class="reference internal" href="Glossary.html#term-synchronization-primitive"><span class="xref std std-term">synchronization primitives</span></a> described in
|
|||
|
|
this chapter can achieve all of these goals. Synchronization primitives are
|
|||
|
|
variable types that have one critical aspect: operations that manipulate the
|
|||
|
|
primitives are guaranteed to be <a class="reference internal" href="Glossary.html#term-atomic"><span class="xref std std-term">atomic</span></a>. This feature contrasts with
|
|||
|
|
standard variables that lack this guarantee. For instance, consider the simple
|
|||
|
|
line of C code <code class="docutils literal notranslate"><span class="pre">x++</span></code> that increments an <code class="docutils literal notranslate"><span class="pre">int</span></code> variable. This line requires three
|
|||
|
|
separate instructions to load the variable into a register, increment the
|
|||
|
|
register, then store the result back into memory. In between these instructions,
|
|||
|
|
the kernel might interrupt the execution and switch to another thread. In
|
|||
|
|
contrast, a synchronization primitive would use special-purpose hardware
|
|||
|
|
techniques to guarantee that this kind of multi-step operation happens in a
|
|||
|
|
single step that cannot be interrupted by the kernel.</p>
|
|||
|
|
<p>Careless misuse of synchronization primitives can cause a variety of problems.
|
|||
|
|
For one thing, some synchronization primitives impose a performance penalty by
|
|||
|
|
turning off all parallel execution in a system. This shutdown requires CPU cores
|
|||
|
|
temporarily save copies of their data and stop running; they may also need to
|
|||
|
|
perform pending writes back to multiple levels of cache. Even worse,
|
|||
|
|
synchronization primitives can lead to <a class="reference internal" href="Glossary.html#term-deadlock"><span class="xref std std-term">deadlock</span></a>, a condition in which
|
|||
|
|
two (or more) threads are simultaneously waiting on each other. Finally, some
|
|||
|
|
synchronization algorithms contain subtle flaws that can be easily overlooked.</p>
|
|||
|
|
<p>The goal for this chapter is to examine the common synchronization primitives
|
|||
|
|
that are widely supported, particular in the POSIX thread library. The focus
|
|||
|
|
here is on the basic intent of each primitive and some principles for avoiding
|
|||
|
|
deadlock and significant performance penalties. In <a class="reference external" href="SynchProblemsOverview.html">Synchronization Problems</a>, we will explore how to combine primitives to
|
|||
|
|
solve more complex problems.</p>
|
|||
|
|
</div>
|
|||
|
|
|
|||
|
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|
|||
|
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</div>
|
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|
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<div class="container">
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|||
|
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<div class="mt-4 container center">
|
|||
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«  <a id="prevmod1" href="Extended6Primes.html">6.8. Extended Example: Concurrent Prime Number Search</a>
|
|||
|
|
  ::  
|
|||
|
|
<a class="uplink" href="index.html">Contents</a>
|
|||
|
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  ::  
|
|||
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<a id="nextmod1" href="CritSect.html">7.2. Critical Sections and Peterson’s Solution</a>  »
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