[dataflow]

There's "data race" in "C++ ISO standard" sense, and then there's "data race" in the general CS literature (as well as all the other terms). Two threads writing a value to the same memory location (even atomically) is usually a data race in the CS/algorithm sense (due to the lack of synchronization), but not the C++ sense. I'm not really interested in pedantic terminology here, just trying get a higher level point across about what you can & can't assume with a clean TSAN (and how not to clean your TSAN errors). Feel free to mentally rewrite my comment with whatever preferred terminology you feel would get my points across.

[Maxatar]

This isn't pedantry, if you're going to talk about how specific tools work then you need to use the actual terminology that the tools themselves use or else you will confuse yourself and anyone you talk to about them. If we were discussing general concepts about thread safety then sure we can be loose about our words, but if we're talking about a specific tool used for a specific programming language then we should make sure we are using the correct terminology, if only to signal that we have the proper domain knowledge to speak about the subject.

>Feel free to mentally rewrite my comment with whatever preferred terminology you feel would get my points across.

If I rewrite your comment to use data race, then your comment is plainly incorrect since the supporting example you give is not a data race but a race condition.

If I rewrite your comment to use race condition, then your comment is also incorrect since TSAN doesn't detect race conditions in general and doesn't claim to, it detects data races.

So what exactly am I supposed to do in order to make sense of your post?

The idea that you'd talk about the pros and cons of a tool like TSAN without knowing the difference between a race condition and a data race is kind of absurd. That you'd claim my clarification of these terms for the sake of better understanding your point is a form of pedantry is sheer hubris.

[dataflow]

Hold on, before attacking me. Say we have this Java program, and assume the semantics of the common JRE/JVM everyone uses. Do you believe it has a data race or not? Because the variable is accessed atomically, whether whether you mark it as volatile or not:

  class Main {
    private static String s = "uninitialized";
    public static void main(String[] args) {
      Thread t = new Thread() {
        public void run() { s = args[0]; }
      };
      t.start();
      System.out.println(s);
    }
  }

And I sure as heck have not heard anyone claim such data races are impossible in Java. (Have you?)

[charleslmunger]

>When a program contains two conflicting accesses (§17.4.1) that are not ordered by a happens-before relationship, it is said to contain a data race.

Yes, your program contains a data race, by the definition used in the JLS. The set of outcomes you may observe from a data race are specified. I'm not sure if this choice was intentional or not, but there is a guarantee that you will either print the argument or "uninitialized" and no other behavior, because String relies on final field semantics. This is would not be true in c/c++ where the equivalent code is undefined behavior and you could see any result.

In Java you can have a data race and use it productively for certain niche cases, like String.hashcode - I've also contributed some to the Guava concurrency library. This is not true in c/c++ where data races (by their definition) are undefined behavior. If you want to do the tricks you can in racy Java without UB, you have to declare your variables atomic and use relaxed memory order and possibly fences.

[tialaramex]

What you wrote is indeed a data race, it'll race s, but you mention the semantics of the JRE and I wonder if you actually know what those are because that's crucial here.

You see Java has a specific memory ordering model (many languages just give you a big shrug, including C before it adopted the C++ 11 model but Java spells out what happens) and that model is very sophisticated so it has an answer to what happens here.

Because we raced s, we lose Sequential Consistency. This means in general (this example is so trivial it won't matter) humans struggle to understand what's going on in their program, which makes debugging and other software engineering impractical. But, unlike C++ loss of Sequential Consistency isn't fatal in Java, instead we're promised that when s is observed in the main thread it will either be that initial "uninitialized" string or it will have the args[0] value, ie the name of the program because these are the only two values it could have and Java does not specify which of them observed in this case.

You could think of this as "atomic access" and that's likely the actual implementation in this case, but the Java specification only promises what I wrote.

In C++ this is game over, the language standard specifically says it is Undefined Behaviour to have any data race and so the behaviour of your program is outside the standard - anything at all might happen.

[Edited: I neglected originally to observe that s is set to "uninitialized", and instead I assumed it begins as null]

[dataflow]

> But, unlike C++ loss of Sequential Consistency isn't fatal in Java

I have no idea what you mean here. Loss of sequential consistency is in no way fatal in C++. There are several access modes that are specifically designed to avoid sequential consistency.

Regarding the rest of your comment:

You're making exactly my point though. These are guaranteed atomic accesses -- and like you said, we are guaranteed to see either the old or new value, and nothing else -- and yet they are still data races. Anyone who agrees this is a data race despite the atomicity must necessarily understand that atomics don't imply lack of data races -- not in general CS terminology.

The only way it's correct to say they are mutually exclusive is when you define "data race" as they did in the C++ standard, to imply a non-atomic access. Which you're welcome to do, but it's an incredibly pedantic thing to do because, for probably >95% of the users of C++ (and probably even of TSAN itself), when they read "data race", they assume it to mean the concept they understand from CS. They don't know that the ISO standard defines it in its own peculiar way. My point here was to convey something to normal people rather than C++ committee language lawyers, hence the use of the general term.

[tialaramex]

> Loss of sequential consistency is in no way fatal in C++. There are several access modes that are specifically designed to avoid sequential consistency.

Sure, if you work really hard you can write a C++ program which doesn't meet the 6.9.2.2 intro.races definition of a data race but does nevertheless lose sequential consistency and so it has in some sense well-defined meaning in C++ but humans can't usefully reason about it. You'll almost certainly trip and write UB when attempting this, but assuming you're inhumanly careful or the program is otherwise very simple so that you don't do that it's an exception.

My guess is that your program will be miscompiled by all extant C++ compilers and you'll be unhappy with the results, and further that if you can get committee focus on this at all they will prioritize making your program Undefined in C++ rather than "fixing" compilers.

Just don't do this. The reason for the exclusions in 6.9.2.2 is that what we want people to do is write correct SC code but using primitives which themselves can't guarantee that, so the person writing the code must do so correctly. The reason is not that somehow C++ programmers are magicians and the loss of SC won't negatively impact the correctness of code they attempt to write, quite the contrary.

[Maxatar]

>The only way it's correct to say they are mutually exclusive is when you define "data race" as they did in the C++ standard, to imply a non-atomic access.

A data-race does not imply a non-atomic operation, it implies an unsynchronized operation. Different languages have different requirements for what constitutes a synchronized operation, for example in Python all reads and writes are synchronized, in Java synchronization is generally accomplished through the use of a monitor or a volatile operation, and in C++ synchronization is generally accomplished through the use of std::atomic operations.

The fact that in C++ atomic operations result in synchronization, while in Java atomic operations are not sufficient for synchronization is not some kind of language lawyering or pedantry, it's because Java makes stronger guarantees about the consequences of a data race versus C++ where a data race can result in any arbitrary behavior whatsoever. As such it should not come as a surprise that the cost of those stronger guarantees is that Java has stronger requirements for data race free programs.

But of course this is mostly a deflection, since the discussion was about the use of TSAN, which is a data race detector for C and C++, not for Java. Hence to the extent that TSAN detects data races, it detects them with respect to C and C++'s memory model, not Java's memory model or Python's memory model, or any other memory model.

The objection I originally laid out was your example of a race condition, an example which can happen even in the absence of parallelism (ie. a single-core CPU) and even in the absence of multithreaded code altogether (your example can happen in a single threaded application with the use of coroutines). TSAN makes no claim with regards to detecting race conditions in general, it only seeks to detect data races and data races as they pertain the C and C++ memory models.

[dataflow]

I am not "deflecting" anything, I am going to the heart of the matter.

Let me lay this out very explicitly. This comment will likely be my final one on the matter as this back-and-forth is getting quite tiresome, and I'm not enjoying it, especially with the swipes directed at me.

Please take a look at the following two C++ and Java programs: https://godbolt.org/z/EjWWac1bG

For the sake of this discussion, assume the command-line arguments behave the same in both languages. (i.e. ignore the C++ argv[0] vs. Java args[0] distinction and whatnot.)

Somehow, you simultaneously believe (a) that Java program contains data races, while believing (b) the C++ program does not.

This is a self-contradictory position. The programs are well-defined, direct translations of each other. They are equivalent in everything but syntax. If one contains a data race, so must the other. If one does not, then neither can the other.

This implies that TSAN does not detect "data races" as it is usually understood in the CS field -- it does not detect anything in the C++ program. What TSAN detects is only a particular, distinct situation that the C++ standard also happens to call a "data race". So if you're talking to a C++ language lawyer, you can say TSAN detects all data races within its window/buffer limits. But if you're talking to someone who doesn't sleep with the C++ standard under their pillow, they're not going to realize C++ is using a language-specific definition, and they're going to assume it flags programs like the equivalent of the Java program above, which has a data race but whose equivalent TSAN would absolutely not flag.

[Maxatar]

The Java specification which can be found here [1] makes clear that with respect to its memory model the following is true:

    1. Per 17.4.5 your example can lead to a data race.

       "When a program contains two conflicting accesses (§17.4.1) that are not ordered by a happens-before relationship, it is said to contain a data race."

    2. Per 17.7 the variable s is accessed atomically.

       "Writes to and reads of references are always atomic, regardless of whether they are implemented as 32-bit or 64-bit values."

However, atomic reads and writes are not sufficient to protect against data races. What atomic reads and writes will protect against is word tearing (outlined in 17.6 where two threads simultaneously write to overlapping parts of the same object with the result being bits from both writes mixed together in memory). However, a data race involving atomic objects can still result in future reads from that object to result in inconsistent values, and this can last indefinitely into the future. This does not mean that reading from a reference will produce a garbage value, but it does mean that two different threads reading from the same reference may end up reading two entirely different objects. So, you can have thread A in an infinite loop repeatedly reading the value "uninitialized" and thread B in another infinite loop repeatedly reading the value args[0]. This can happen because both threads have their own local copy of the reference which will never be updated to reflect a consistent shared state.

As per 17.4.3, a data-race free program will not have this kind of behavior where two threads are in a perpetually inconsistent state, as the spec says "If a program has no data races, then all executions of the program will appear to be sequentially consistent."

So while atomicity protects against certain types of data corruption, it does not protect against data races.

[1] https://docs.oracle.com/javase/specs/jls/se24/html/jls-17.ht...

[kiitos]

> Two threads writing a value to the same memory location (even atomically) is usually a data race in the CS/algorithm sense (due to the lack of synchronization), but not the C++ sense

You seem to conflate the concepts of "data race" and "race condition", which are not the same thing.

Two threads writing to the same memory location without synchronization (without using atomic operations, without going thru a synchronization point like a mutex, etc.) is a data race, and almost certainly also a race condition. If access to that memory location is synchronized, whether thru atomics or otherwise, then there's no data race, but there can still be a race condition.

This isn't a pedantic distinction, it's actually pretty important.

[vlovich123]

> Two threads writing a value to the same memory location (even atomically) is usually a data race in the CS/algorithm sense (due to the lack of synchronization), but not the C++ sense

Not only are you incorrect, it’s even worse than you might think. Unsynchronized access to data in c++ is not only a data race, it’s explicitly undefined behavior and the compiler can choose to do whatever in response of an observed data race (which you are promising it isn’t possible by using the language).

You are also misinformed about the efficacy of TSAN. Even in TSAN you have to run it in a loop - if TSAN never observes the specific execution order in a race it’ll remain silent. This isn’t a theoretical problem but a very real one you must deeply understand if you rely on these tools. I recall a bug in libc++ with condition_variable and reproducing it required running the repro case in a tight loop like a hundred times to get even one report. And when you fixed it, how long would you run to have confidence it was actually fixed?

And yes, race conditions are an even broader class of problems that no tool other than formal verification or DST can help with. Hypothesis testing can help mildly but really you want at least DST to probabilistically search the space to find the race conditions (and DST’s main weakness aside from the challenge of searching a combinatorial explosion of states is that it still relies on you to provide test coverage and expectations in the first place that the race condition might violate)

[manwe150]

TSAN observes the lack of an explicit order and warns about that, so it is better in some sense than just running normally in a loop and hoping to see the occurrence of a specific mis-ordering. But that part of it is a data race detector, so it cannot do anything for race conditions, and as soon as something is annotated as atomic, it cannot do anything to detect misuse. It can be better for lock evaluation, as it can check they are always acquired in the same order without needing to actually observe a conflicting deadlock occurring. But I agree you need more formal tooling to actually show the problem is eliminated and not just improbable

[dataflow]

Geez. I'm well aware it's UB. That was just sloppy wording. Should've said "not necessarily", okay. I only wrote "not in the C++ sense" because I had said "even atomically", not because I'm clueless.