8 ms·

> Overall Rust appears to be fulfilling one of its original design goals: allowing us to write more concurrent code safely. Both WebRender and Stylo are very la

by est31 6y ago

> Overall Rust appears to be fulfilling one of its original design goals: allowing us to write more concurrent code safely. Both WebRender and Stylo are very large and pervasively multi-threaded, but have had minimal threading issues. What issues we did find were mistakes in the implementations of low-level and explicitly unsafe multithreading abstractions — and those mistakes were simple to fix. > This is in contrast to many of our C++ races, which often involved things being randomly accessed on different threads with unclear semantics, necessitating non-trivial refactorings of the code.

chowells 6y ago
Yeah, that was the part that stood out to me as well. I thought it was pretty intuitively obvious that having your data races confined to small segments is better than allowing them to be spread over the entire codebase, but a lot of people act like it negates the advantage of working in a safer language entirely. It's nice to have empirical reports that back up the "obvious" though. Sometimes "obvious" things turn out to be wrong. It's always good to check them.
- setr 6y ago
  > but a lot of people act like it negates the advantage of working in a safer language entirely. I think the underlying assumption is more along the lines of: the easy cases are made easier, and the hard cases are made impossible — or rather the concern being that rust hasn’t made the problem easier, it just moved it around. Which is often the case with claims of solving long-lasting problems
  - kibwen 6y ago
    > the concern being that rust hasn’t made the problem easier, it just moved it around. This is largely the point of Rust, though. Rust takes problems that used to manifest at runtime and causes them to manifest at compile-time, shortening the feedback loop drastically. It also takes problems that used to manifest all over the codebase and roots their causes in the leaf nodes of the program that are making use of `unsafe` blocks. Moving around the problem turns out to be a valuable way of addressing the problem, without ignoring any of the essential complexity or making anything fundamentally impossible.
    - wtetzner 6y ago
      > shortening the feedback loop drastically More importantly, forcing you to catch them. Problems at runtime don't get caught until that condition actually occurs during an actual run of the software.
  - pdpi 6y ago
    Not all forms of moving things around are equivalent. Some just keep the problem as diffuse as it was before, others restrict the problem to a smaller area. Rust does not, by any means, solve this problem. What it does do is help you prove that the problem is contained to a smaller subset of your codebase that you can audit/reason about much more easily.
  - chriswarbo 6y ago
    > Which is often the case with claims of solving long-lasting problems This is what I really like about programming language theory: problems which are literally undecidable in existing languages, can be made almost trivial by designing a language with that problem in mind. The trick is to have the language keep track of the information needed for the solution, so we're not dealing with "black boxes". A classic example is annotating every variable with a type, and propagating that information through compound expressions; lets us spot problems like trying to use a number as a string, without worrying about the halting problem (we do this by forbidding some technically-valid programs, like `1 + (true? 10 : "hello")`). Of course, this leaves the much harder problems of (a) keeping the burden imposed by this extra information down to a reasonable level, and (b) trying to make the language compelling enough for people to actually use. Rust is one of the few examples of going all-in on a new ecosystem, and it actually being embraced. Often it's easier to "embed" a new language inside an existing language, to piggy-back on the existing tooling (e.g. the variety of auto-differentiating languages built in Python)
    - Quekid5 6y ago
      Yes, people often forget that TMs really are "pure runtime". Sure, the program itself is static, but nothing else is. There's no imposed semantics on anything beyond the pure mechanism of the interpreter itself. This lessens the 'problem' part of the Halting Problem drastically if you just impose some constraints on the programs, e.g. static types, maybe a borrow-checker, etc.
  - stouset 6y ago
    > I think the underlying assumption is more along the lines of: the easy cases are made easier, and the hard cases are made impossible — or rather the concern being that rust hasn’t made the problem easier, it just moved it around. Except in this case the article linked demonstrates that they've made things dramatically easier overall. Data races went from being endemic in a C++ codebase to being in a few low-level and easy-to-locate areas. The easy stuff stayed easy, the bulk of the rest was folded into the easy case, and even the hard cases appear to have been made easier to find, identify, and fix.
    - Quekid5 6y ago
      Once you pick away enough at the straws, the needle becomes a lost easier to find.
kibwen 6y ago
And when it comes to finding bugs in your unsafe concurrency primitives in Rust programs, there is a tool called Loom that helps with that, as a powerful complement to what ThreadSanitizer provides: https://github.com/tokio-rs/loom https://github.com/tokio-rs/loom . You can think of Loom sort of like QuickCheck, except the permutations are all the possible thread states of your program based on the memory orderings that you have specified. Here's a video that briefly talks about what Loom can do https://youtu.be/rMGWeSjctlY?t=7519 https://youtu.be/rMGWeSjctlY?t=7519 (it's part of a larger video about memory orderings, and is a great introduction to the topic).
- gamegoblin 6y ago
  Loom is really awesome, though it is focused on exhaustive testing, so not suitable for code that has a lot of possible interleavings (e.g. due to a ton of threads, or a large body of code). There is a new project out of AWS called Shuttle [1] which is like Loom, but it does random exploration instead of exhaustive exploration, which enables massively distributed testing of really complicated stuff. [1] https://github.com/awslabs/shuttle https://github.com/awslabs/shuttle
  - kibwen 6y ago
    Excellent, thanks for the tip! I have to ask though, given a tool that does exhaustive testing (like Loom), shouldn't it be trivial to adapt it to do random testing merely by randomizing rather than enumerating the test inputs? Is there something more going on that I'm not seeing?
  - wyldfire 6y ago
    Is there anything like Loom or Shuttle for C/C++? Or are these only implemented in rust and they'd still work with C/C++?
    - kibwen 6y ago
      I vaguely recall that Loom is an implementation of a research paper that purported to implement this for C/C++, but I don't know if that implementation is available/production-ready. In the worst case it should be relatively straightforward to back-port Loom to C++, since Rust also uses the C memory model.
evmar 6y ago
It's interesting, I read this and had the opposite conclusion -- people are still writing racy code using Rust. I mean, it's great that you can write threaded code with more confidence, but it seems the conclusion here is that Rust gives you more confidence but not absolute confidence and you still need the normal stable of runtime detection tooling. The second fix[1] in particular resembles the bad pattern of "sprinkle atomics it until it works" you see in non-Rust languages. (If this comment raises your hackles, please read the "no true Scotsman[2]" fallacy before responding.) [1]: https://hg.mozilla.org/integration/autoland/rev/044f8b594c71 https://hg.mozilla.org/integration/autoland/rev/044f8b594c71 [2]: https://en.wikipedia.org/wiki/No_true_Scotsman https://en.wikipedia.org/wiki/No_true_Scotsman
- brundolf 6y ago
  > What issues we did find were mistakes in the implementations of low-level and explicitly unsafe multithreading abstractions (emphasis mine) The important thing is that the races only happened in unsafe { } blocks. It's well-established that these blocks should basically be treated like C++ in terms of scrutiny, but (I believe) you can still have roughly "absolute confidence" in non-unsafe code. It's true and interesting that unsafe { } blocks deserve sanitization analysis like C++ code does - and maybe this hasn't been discussed enough - but I don't think it's fair to suggest that the enormous language benefits are outweighed by some false sense of confidence. The proportions in that idea just seem way out of whack.
  - benschulz 6y ago
    AFAIU one can create races in safe code using atomics with insufficient ordering. (That notwithstanding, I agree that Rust is a vast improvement over C/++.)
    - steveklabnik 6y ago
      "races" is too general. Safe Rust cannot have data races. It can have race conditions.
WalterBright 6y ago
D takes a different approach. D has a type constructor `shared`. Only types marked as `shared` (or `immutable`) can be accessed by multiple threads. If you've got concurrency problems, the scope of the problems are immediately reduced to the subset of the code typed as `shared`. In addition, `shared` types cannot be accessed directly. They can only be accessed via library routines, such as D's atomic library. Having an `immutable` data type also helps, as immutable data can be accessed by multiple threads without need for synchronization.
- eximius 6y ago
  It's not that different. Rust has Sync + Send + &refs and &mut refs.
- secondcoming 6y ago
  Does this approach have to deal with potential recursive locking?
  - WalterBright 6y ago
    Not by itself. But D's prototype ownership-borrowing system can help with that.
- d110af5ccf 6y ago
  Did D ever get around to specifying a formal memory model? Because when I tried to adopt D a couple years ago I felt there was a lot of ambiguity surrounding the semantics of `shared`, particularly when interfacing with C and C++ code. I ended up just casting to and from `shared`, and that seemed to work, but it was pretty verbose for highly parallel code and I was never quite sure under what circumstances doing that might violate the compiler's invariants. Also such casting appeared to eliminate most of the benefits, since what appeared to be local data might have been cast and had a pointer sent across to another thread elsewhere. In the end `shared`, @nogc antics (the language internals weren't fully annotated), closures requiring the GC (compare to C++), and the friction of interfacing with highly templated C++ code such as the Eigen library caused me to abandon the attempt. I sure learned a lot in the process though!
deleted 6y ago
[deleted]

by est31 6y ago

chowells 6y ago

Yeah, that was the part that stood out to me as well. I thought it was pretty intuitively obvious that having your data races confined to small segments is better than allowing them to be spread over the entire codebase, but a lot of people act like it negates the advantage of working in a safer language entirely. It's nice to have empirical reports that back up the "obvious" though. Sometimes "obvious" things turn out to be wrong. It's always good to check them.

setr 6y ago
> but a lot of people act like it negates the advantage of working in a safer language entirely. I think the underlying assumption is more along the lines of: the easy cases are made easier, and the hard cases are made impossible — or rather the concern being that rust hasn’t made the problem easier, it just moved it around. Which is often the case with claims of solving long-lasting problems
- kibwen 6y ago
  > the concern being that rust hasn’t made the problem easier, it just moved it around. This is largely the point of Rust, though. Rust takes problems that used to manifest at runtime and causes them to manifest at compile-time, shortening the feedback loop drastically. It also takes problems that used to manifest all over the codebase and roots their causes in the leaf nodes of the program that are making use of `unsafe` blocks. Moving around the problem turns out to be a valuable way of addressing the problem, without ignoring any of the essential complexity or making anything fundamentally impossible.
  - wtetzner 6y ago
    > shortening the feedback loop drastically More importantly, forcing you to catch them. Problems at runtime don't get caught until that condition actually occurs during an actual run of the software.
- pdpi 6y ago
  Not all forms of moving things around are equivalent. Some just keep the problem as diffuse as it was before, others restrict the problem to a smaller area. Rust does not, by any means, solve this problem. What it does do is help you prove that the problem is contained to a smaller subset of your codebase that you can audit/reason about much more easily.
- chriswarbo 6y ago
  > Which is often the case with claims of solving long-lasting problems This is what I really like about programming language theory: problems which are literally undecidable in existing languages, can be made almost trivial by designing a language with that problem in mind. The trick is to have the language keep track of the information needed for the solution, so we're not dealing with "black boxes". A classic example is annotating every variable with a type, and propagating that information through compound expressions; lets us spot problems like trying to use a number as a string, without worrying about the halting problem (we do this by forbidding some technically-valid programs, like `1 + (true? 10 : "hello")`). Of course, this leaves the much harder problems of (a) keeping the burden imposed by this extra information down to a reasonable level, and (b) trying to make the language compelling enough for people to actually use. Rust is one of the few examples of going all-in on a new ecosystem, and it actually being embraced. Often it's easier to "embed" a new language inside an existing language, to piggy-back on the existing tooling (e.g. the variety of auto-differentiating languages built in Python)
  - Quekid5 6y ago
    Yes, people often forget that TMs really are "pure runtime". Sure, the program itself is static, but nothing else is. There's no imposed semantics on anything beyond the pure mechanism of the interpreter itself. This lessens the 'problem' part of the Halting Problem drastically if you just impose some constraints on the programs, e.g. static types, maybe a borrow-checker, etc.
- stouset 6y ago
  > I think the underlying assumption is more along the lines of: the easy cases are made easier, and the hard cases are made impossible — or rather the concern being that rust hasn’t made the problem easier, it just moved it around. Except in this case the article linked demonstrates that they've made things dramatically easier overall. Data races went from being endemic in a C++ codebase to being in a few low-level and easy-to-locate areas. The easy stuff stayed easy, the bulk of the rest was folded into the easy case, and even the hard cases appear to have been made easier to find, identify, and fix.
  - Quekid5 6y ago
    Once you pick away enough at the straws, the needle becomes a lost easier to find.

kibwen 6y ago

And when it comes to finding bugs in your unsafe concurrency primitives in Rust programs, there is a tool called Loom that helps with that, as a powerful complement to what ThreadSanitizer provides: https://github.com/tokio-rs/loom https://github.com/tokio-rs/loom . You can think of Loom sort of like QuickCheck, except the permutations are all the possible thread states of your program based on the memory orderings that you have specified. Here's a video that briefly talks about what Loom can do https://youtu.be/rMGWeSjctlY?t=7519 https://youtu.be/rMGWeSjctlY?t=7519 (it's part of a larger video about memory orderings, and is a great introduction to the topic).

gamegoblin 6y ago
Loom is really awesome, though it is focused on exhaustive testing, so not suitable for code that has a lot of possible interleavings (e.g. due to a ton of threads, or a large body of code). There is a new project out of AWS called Shuttle [1] which is like Loom, but it does random exploration instead of exhaustive exploration, which enables massively distributed testing of really complicated stuff. [1] https://github.com/awslabs/shuttle https://github.com/awslabs/shuttle
- kibwen 6y ago
  Excellent, thanks for the tip! I have to ask though, given a tool that does exhaustive testing (like Loom), shouldn't it be trivial to adapt it to do random testing merely by randomizing rather than enumerating the test inputs? Is there something more going on that I'm not seeing?
- wyldfire 6y ago
  Is there anything like Loom or Shuttle for C/C++? Or are these only implemented in rust and they'd still work with C/C++?
  - kibwen 6y ago
    I vaguely recall that Loom is an implementation of a research paper that purported to implement this for C/C++, but I don't know if that implementation is available/production-ready. In the worst case it should be relatively straightforward to back-port Loom to C++, since Rust also uses the C memory model.

evmar 6y ago

It's interesting, I read this and had the opposite conclusion -- people are still writing racy code using Rust. I mean, it's great that you can write threaded code with more confidence, but it seems the conclusion here is that Rust gives you more confidence but not absolute confidence and you still need the normal stable of runtime detection tooling. The second fix[1] in particular resembles the bad pattern of "sprinkle atomics it until it works" you see in non-Rust languages. (If this comment raises your hackles, please read the "no true Scotsman[2]" fallacy before responding.) [1]: https://hg.mozilla.org/integration/autoland/rev/044f8b594c71 https://hg.mozilla.org/integration/autoland/rev/044f8b594c71 [2]: https://en.wikipedia.org/wiki/No_true_Scotsman https://en.wikipedia.org/wiki/No_true_Scotsman

brundolf 6y ago
> What issues we did find were mistakes in the implementations of low-level and explicitly unsafe multithreading abstractions (emphasis mine) The important thing is that the races only happened in unsafe { } blocks. It's well-established that these blocks should basically be treated like C++ in terms of scrutiny, but (I believe) you can still have roughly "absolute confidence" in non-unsafe code. It's true and interesting that unsafe { } blocks deserve sanitization analysis like C++ code does - and maybe this hasn't been discussed enough - but I don't think it's fair to suggest that the enormous language benefits are outweighed by some false sense of confidence. The proportions in that idea just seem way out of whack.
- benschulz 6y ago
  AFAIU one can create races in safe code using atomics with insufficient ordering. (That notwithstanding, I agree that Rust is a vast improvement over C/++.)
  - steveklabnik 6y ago
    "races" is too general. Safe Rust cannot have data races. It can have race conditions.

WalterBright 6y ago

D takes a different approach. D has a type constructor `shared`. Only types marked as `shared` (or `immutable`) can be accessed by multiple threads. If you've got concurrency problems, the scope of the problems are immediately reduced to the subset of the code typed as `shared`. In addition, `shared` types cannot be accessed directly. They can only be accessed via library routines, such as D's atomic library. Having an `immutable` data type also helps, as immutable data can be accessed by multiple threads without need for synchronization.

eximius 6y ago
It's not that different. Rust has Sync + Send + &refs and &mut refs.
secondcoming 6y ago
Does this approach have to deal with potential recursive locking?
- WalterBright 6y ago
  Not by itself. But D's prototype ownership-borrowing system can help with that.
d110af5ccf 6y ago
Did D ever get around to specifying a formal memory model? Because when I tried to adopt D a couple years ago I felt there was a lot of ambiguity surrounding the semantics of `shared`, particularly when interfacing with C and C++ code. I ended up just casting to and from `shared`, and that seemed to work, but it was pretty verbose for highly parallel code and I was never quite sure under what circumstances doing that might violate the compiler's invariants. Also such casting appeared to eliminate most of the benefits, since what appeared to be local data might have been cast and had a pointer sent across to another thread elsewhere. In the end `shared`, @nogc antics (the language internals weren't fully annotated), closures requiring the GC (compare to C++), and the friction of interfacing with highly templated C++ code such as the Eigen library caused me to abandon the attempt. I sure learned a lot in the process though!

deleted 6y ago

[deleted]