[dataflow]

> C++ doesn't consider two atomic accesses of the same memory (at least one being a write) under memory_order_relaxed to be a data race, but TSAN does.

I'm confused what you're saying here. If you take the program I linked, which uses relaxed ordering, and add -fsanitize=thread, TSAN doesn't flag anything. That seems inconsistent with what you're saying?

P.S. note that even using memory_order_seq_cst wouldn't change anything in that particular program.

[kiitos]

First of all, TSAN can identify the presence of a data race, but it can't prove the absence of any data races. If -fsanitize=thread doesn't flag anything, that's insufficient evidence to say that there aren't any data races in the code, at least as TSAN defines data race, which is stricter than how C++ defines data race.

You've now received many comments from many different commenters that all kind of say the same thing, which is basically that your understanding of a "data race" is not really accurate. Those comments have included pretty detailed information as to exactly how and when and why your definition isn't totally correct. If I were you I'd take the L and maybe read up a bit.

[dataflow]

> First of all, TSAN can identify the presence of a data race, but it can't prove the absence of any data races. If -fsanitize=thread doesn't flag anything, that's insufficient evidence to say that there aren't any data races in the code

I stated as much in my own first comment, with more details on when/why this does/doesn't occur.

> at least as TSAN defines data race, which is stricter than how C++ defines data race. [...] If I were you I'd take the L and maybe read up a bit.

Before assuming I haven't: I have. And the reading does not agree [1] [2] with your idea that TSAN sometimes considers relaxed atomic writes to be data races. Hence my replies. Remember, you wrote:

>> C++ doesn't consider two atomic accesses of the same memory (at least one being a write) under memory_order_relaxed to be a data race, but TSAN does.

I have not seen a single word anywhere -- not in the docs, not in example code, and (I think) not even from anyone here other than you -- suggesting that TSAN considers memory_order_relaxed writes to constitute a data race. It certainly does not flag them in the most trivial test I could think of, which I had already linked here [3].

If this is just my ignorance or misunderstanding, then instead of telling me to go do my own reading, please enlighten me and provide one link or example that demonstrates that TSAN considers atomic writes to the same memory to be a data race? I would be very glad to learn this, as I wasn't aware of this before, and am not seeing anything suggesting such.

> Those comments have included pretty detailed information as to exactly how and when and why your definition isn't totally correct.

I have linked to papers going back decades showing that "data race" has a general definition that don't entirely match up with what people have said. I myself have also explained in great detail how the general definition differs from the C++ one. I don't know what else to possibly provide here, but I'm done.

[1] https://clang.llvm.org/docs/ThreadSanitizer.html

[2] https://github.com/google/sanitizers/wiki/threadsanitizercpp...

[3] https://godbolt.org/z/EjWWac1bG

[reitzensteinm]

I haven't participated in this thread yet, but I would like to drill down on your TSan example. It seems to me that the window of timing for TSan to catch it is _super_ tight, as the overhead of creating a thread is very large relative to the other operations.

For something like TSan, which allows programs to execute normally with additional instrumentation, this timing matters, and so it's not a great example. An equivalent program being simulated in something like Loom would be much more convincing.

I'm a little confused, as you agree with your parent commenter that TSan not raising a flag is not conclusive. But you also appear to be using TSan not flagging the program as some kind of evidence in the same comment.

[dataflow]

Thanks for following along and trying to clarify this, I really appreciate it.

The answer to your questions is that timing is not the issue in my example. You can notice this easily if you strip std::atomic<> from the type. TSAN can and does catch it just fine. The atomicity itself is what tells TSAN to not consider this a data race.

What probably threw you off was that I (sloppily) used "timing" as a way to say "proximity in the history buffer". [1] It's not wall clock time that matters, it's the number of memory accesses that fit in TSAN's history buffer. (This should also explain your confusion w.r.t. "tight" timing.)

Hence, the conclusivity depends entirely on the reason it wasn't flagged. (This is why I explained the failure modes quite precisely in my very first comment: not all of them have randomness involved.) If it wasn't flagged because the history buffer wasn't large enough, then obviously it's not conclusive. But if it wasn't flagged because TSAN noticed it and deliberately exempted it, then obviously it doesn't consider it a data race.

[1] https://github.com/google/sanitizers/wiki/ThreadSanitizerFla...

[kiitos]

> The answer to your questions is that timing is not the issue in my example. You can notice this easily if you strip std::atomic<> from the type. TSAN can and does catch it just fine.

If you strip the std::atomic from your example, then you obviously lose read/write atomicity on the value, which should be trivial for something like TSAN to detect and flag.

> The atomicity itself is what tells TSAN to not consider this a data race ... the conclusivity depends entirely on the reason it wasn't flagged. (This is why I explained the failure modes quite precisely in my very first comment: not all of them have randomness involved.)

"The conclusivity" can only ever be "inconclusive" or "has a data race", it can never be "does not have a data race", because that's just not how TSAN (or any similar tools) work. See [1]. In your original C++ program there is no happens-before relationship between the thread you spawn and the main thread, so there is a data race by the TSAN definition, even though the read and the write are atomic, and even if a given execution of that code by TSAN doesn't yield an error. It's not about timing, at least not exactly -- it's about the guarantees of the scheduler and its execution of threads, which is non-deterministic without explicit synchronization in the application (or something along those lines)..!

[1] https://github.com/google/sanitizers/wiki/ThreadSanitizerAbo...

[reitzensteinm]

Thanks for expanding, I was misunderstanding the point you were trying to make with the example.

[dataflow]

Yup, glad I could clarify!

[kiitos]

You've linked to papers that define "data race" in a specific way. That doesn't mean that when anyone says "data race" in any other context they are using any of those papers' definitions, for reasons that have been exhaustively explained.