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Questions tagged [memory-visibility]

35 questions
210
votes
5 answers

Why does this Java program terminate despite that apparently it shouldn't (and didn't)?

A sensitive operation in my lab today went completely wrong. An actuator on an electron microscope went over its boundary, and after a chain of events I lost $12 million of equipment. I've narrowed down over 40K lines in the faulty module to…
Dog
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100
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7 answers

Are static variables shared between threads?

My teacher in an upper level Java class on threading said something that I wasn't sure of. He stated that the following code would not necessarily update the ready variable. According to him, the two threads don't necessarily share the static…
dontocsata
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15
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2 answers

Volatile variables and other variables

The following is from classical Concurency in Practice: When thread A writes to a volatile variable and subsequently thread B reads the same variable, the values of all variables that were visible to A prior to writing to the volatile…
Cratylus
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14
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4 answers

in what architectures/OS other thread can see default nonfinal field values after constructor call?

I'm trying to reproduce a memory visibility issue in case of insufficient object initialization for non-final fields (JLS 17.5 Final Field Semantics, FinalFieldExample class example). Where it stated "However, f.y is not final; the reader() method…
yetanothercoder
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14
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3 answers

Java ConcurrentHashMap.computeIfPresent value modification visibility

Let's say I have a concurrent map with collections as value: Map map = new ConcurrentHashMap<>(); map.putIfAbsent(8, new ArrayList<>()); and I update the value as follows: map.computeIfPresent(8, (i, c) -> { c.add(5); …
Marin Veršić
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12
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5 answers

Is unsynchronized read of integer threadsafe in java?

I see this code quite frequently in some OSS unit tests, but is it thread safe ? Is the while loop guaranteed to see the correct value of invoc ? If no; nerd points to whoever also knows which CPU architecture this may fail on. private int invoc =…
krosenvold
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12
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2 answers

Does atomic variables guarantee memory visibility?

Small question about memory visibility. CodeSample1: class CustomLock { private boolean locked = false; public boolean lock() { if(!locked) { locked = true; return true; } return false; …
JamesJenkins
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6
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3 answers

Visibility Guarantee

I have read a few explanations of section 16.3 "Initialization Safety" from JCIP and am still not clear. The section states that "Further, any variables that can be reached through a final field of a properly constructed object (such as the elements…
CaptainHastings
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5
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4 answers

jvm reordering/visibility effect test

While writing some java article I'm trying to reproduce re-ordering in case of unsynchronized object costruction in multithreaded environment. The case when a heavy object is constructed w/o synchonization/volatiles/finals and other threads get…
yetanothercoder
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5
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3 answers

Atomic operation propagation/visibility (atomic load vs atomic RMW load)

Context  I am writing a thread-safe protothread/coroutine library in C++, and I am using atomics to make task switching lock-free. I want it to be as performant as possible. I have a general understanding of atomics and lock-free programming, but I…
RmbRT
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4
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0 answers

Visibility guarantee with CompletableFuture's thenAccept

I'm trying to understand if there are any visibility-guarantees provided by CompletableFuture. Suppose I've a class called SampleClass which is something like the following: public class SampleClass { private String member1; private String…
Lavish Kothari
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4
votes
2 answers

C++11 atomics. visibility and thread.join() / correct way to stop a thread

For which (if any?) STORE_ORDER & LOAD_ORDER does C++11 guarantee that this code runs in finite time? std::atomic a{false}; std::thread t{[&]{ while(!a.load(LOAD_ORDER)); }}; a.store(true, STORE_ORDER); t.join(); I see two issues with…
Chronial
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4
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0 answers

How to demonstrate memory visibility problems in Go?

I'm giving a presentation about the Go Memory Model. The memory model says that without a happens-before relationship between a write in one goroutine, and a read in another goroutine, there is no guarantee that the reader will observe the…
stephenbez
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4
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1 answer

If Thread B wishes to see changes Thread A makes, can only the last change be to a volatile variable as opposed to all?

I've looked at this answer, and it states how: Under the new memory model, when thread A writes to a volatile variable V, and thread B reads from V, any variable values that were visible to A at the time that V was written are guaranteed now to…
Mario Ishac
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4
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3 answers

Is volatile needed?

if I have a byte queue, where it is expected to have one thread producer, another consumer: class ByteQueue{ byte[] buf; /*volatile?*/ int readIdx; /*volatile?*/ int writeIdx; Runnable writeListener; Runnable readListener; //…
fbenoit
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