16 ms·

The occurrence of data races depends on the specific non-deterministic sequence of execution of concurrent codepaths. Just because you have 100% code coverage d

The occurrence of data races depends on the specific non-deterministic sequence of execution of concurrent codepaths. Just because you have 100% code coverage does not mean you've covered every potential execution sequence, and its almost never practical to actually execute every possibility to ensure the absence of data races. Depending on the probability that your data race will occur, it could indeed be something you have to make stars align for TSAN to catch. Not to talk my own book, but there is a well-known alternative to C++ that can actually guarantee the absence of data races.

dataflow 1y ago
It "could" for some algorithms, yes, but for a lot of algorithms, that kind of star alignment simply isn't necessary to find all the data races, was my point. And yes, TLA+ etc. can be helpful, but then you have the problem of matching them up with the code.
- Maxatar 1y ago
  I feel like in a subtle way you're mixing up data races with race conditions, especially given the example you site about incrementing an atomic variable. TSAN does not check for race conditions in general, and doesn't claim to do so at all as the documentation doesn't include the term race condition anywhere. TSAN is strictly for checking data races and deadlocks. Consequently this claim is false: >The issue is that even if it statically proved the absence of data races in the C++ sense, that still wouldn't imply that your algorithm is race-free. Race-free code means absence of data races, it does not mean absence of the more general race condition. If you search Google Scholar for race free programming you'll find no one uses the term race-free to refer to complete absence of race conditions but rather to the absence of data races.
  - dataflow 1y ago
    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.
    - kiitos 1y ago
      > 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.
    - Maxatar 1y ago
      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 1y ago
        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?)
- withoutboats3 1y ago
  The alternative to C++ that I meant was Rust, which statically prevents data races.
  - kiitos 1y ago
    ...so long as your code doesn't use unsafe, neither directly nor transitively.

dataflow 1y ago

It "could" for some algorithms, yes, but for a lot of algorithms, that kind of star alignment simply isn't necessary to find all the data races, was my point. And yes, TLA+ etc. can be helpful, but then you have the problem of matching them up with the code.

Maxatar 1y ago
I feel like in a subtle way you're mixing up data races with race conditions, especially given the example you site about incrementing an atomic variable. TSAN does not check for race conditions in general, and doesn't claim to do so at all as the documentation doesn't include the term race condition anywhere. TSAN is strictly for checking data races and deadlocks. Consequently this claim is false: >The issue is that even if it statically proved the absence of data races in the C++ sense, that still wouldn't imply that your algorithm is race-free. Race-free code means absence of data races, it does not mean absence of the more general race condition. If you search Google Scholar for race free programming you'll find no one uses the term race-free to refer to complete absence of race conditions but rather to the absence of data races.
- dataflow 1y ago
  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.
  - kiitos 1y ago
    > 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.
  - Maxatar 1y ago
    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 1y ago
      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?)
withoutboats3 1y ago
The alternative to C++ that I meant was Rust, which statically prevents data races.
- kiitos 1y ago
  ...so long as your code doesn't use unsafe, neither directly nor transitively.