[mumblemumble]

In a lot of code I've seen, there are threads polling some variable without using any sort of special guard. The assumption (based, I assume, on how you really could get away with this back in the days of single-core, single-CPU computers) is that you only need to worry about race conditions when writing to primitive variables, and that simply reading them is always safe.

[Kranar]

Okay but the poster mentioned a mutex, which would not be a good way to go about polling a variable in Java. All you need to guarantee synchronization of primitive values in Java is the use of volatile [1]. If you need to compose atomic operations together, then you can use an atomic or a mutex, but it would not occur to me to use a mutex to perform a single atomic read or write on a variable in Java.

[1] https://docs.oracle.com/javase/specs/jls/se8/html/jls-8.html...

[mumblemumble]

> All you need to guarantee synchronization of primitive values in Java is the use of volatile

I think I know what you mean, but that's a very dangerous way to word it when speaking in public. It would be more correct to say that "all you need to guarantee reads are protected by memory barriers is volatile."

The distinction matters because, to someone who doesn't already know all about volatile, the way you worded it might lead them to believe that `x++;` is an atomic statement if x is volatile, which is not true. That's a specific example of where things like atomic types are necessary.

(For the curious: https://www.baeldung.com/java-atomic-variables)

I think maybe what you're missing about what I'm saying is that I'm trying to mainly talk for the benefit of people who don't have a solid understanding of how to do safe and performant multithreading. Which is the vast majority of programmers. For that sort of audience, I tend to agree with dragontamer that "just use a mutex" is probably the safest advice to start out. Producing results faster doesn't count for much if you're producing wrong results faster.

[dragontamer]

Java is somewhat cheating, because it got its memory model figured out years before other languages like C or C++.

In C++, you'd have to use OS-specific + compiler-specific routines like InterlockedIncrement64 to get guarantees about when or how it was safe to read/write variables.

Not anymore of course: C++11 provides us with atomic-load and atomic-store routines with the proper acquire / release barriers (and seq-cst default access very similar to Java's volatile semantics).

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Anyway, put yourself into the mindset of a 2009-era C++ programmer for a sec. InterlockedIncrement works on Windows but not in Linux. You got atomics on GCC, but they don't work the same as Visual Studio atomics.

Answer: Mutex lock and mutex-unlock. And then condition variables for polling. Yeah, its slower than InterlockedIncrement / seq-cst atomic variables with proper memory barriers, but it works and is reasonably portable. (Well, CriticalSections on Windows because I never found a good pthreads library for Windows)

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Its still relevant because you still see these thread issues come up in old C++ code.

[Kranar]

I don't understand the relevance of your point. The point I originally asked for clarification about was the use of a mutex for a "polling variable".

Java has had volatile variables since the year 2000, I don't see how it's cheating that Java provided a standardized way of accessing a synchronized value before C and C++ did. Can you elaborate on your point that it's cheating?

In C and C++, for 10 years now, there is a standard library providing atomic data types and atomic instructions. Prior to the standardization one used platform specific atomic facilities. boost has provided cross-platform atomic operations that work on virtually every platform since 2002. Prior to 2002 there were no multicore x86 processors. There would have been mainframe computers that were multicore, is it your argument that code written for those mainframes are of relevant use today by fairly typical C and C++ developers?

At any rate, at no point did any of Java, C, or C++ require the use of a mutex in order to properly synchronize access to a "polling variable". Atomic operations were widely available to all three languages in various ways and would have been the preferred method.

[gpderetta]

While agree with your general point, there were multiprocessor x86 systems well before 2002. Dual and four socket systems were relatively common and the like of SGI and HP would have been happy to sell you x86 systems with even higher socket counts.