I appreciate the sentiment about prior art, but I'm already familiar with both of those links and a significant part of my day job involves working on concurrency frameworks. Specific use cases are more what I'm after. For instance, what would you like to use mutexes for?
On Sat, Jan 18, 2020 at 2:34 AM Alexis King <lexi.lam...@gmail.com> wrote: > Oh, an addendum: I would be remiss not to mention the excellent paper on > the design of Haskell’s asynchronous exception system, which provides both > examples of problems in the wild and more general elaboration on both the > design space and the particular point within it the authors chose for > Haskell. The paper is “Asynchronous Exceptions in Haskell” by Marlow, > Peyton Jones, Moran, and Reppy, and it is available here: > > > https://www.microsoft.com/en-us/research/wp-content/uploads/2016/07/asynch-exns.pdf > > Another thing worth reading is this recent blog post by Simon Marlow (the > first author of the aforementioned paper) on asynchronous exceptions: > > https://simonmar.github.io/posts/2017-01-24-asynchronous-exceptions.html > > On Jan 18, 2020, at 04:27, Alexis King <lexi.lam...@gmail.com> wrote: > > I don’t personally have any problems with Racket’s semaphore interface as > it exists today. I think having the choice of whether or not to enable > breaks mostly makes sense as something the ambient environment controls, > not individual pieces of synchronization logic, since you usually want > control structures like `with-handlers` and `dynamic-wind` to be the things > that mask interrupts in the appropriate places. A hypothetical > `with-critical-section` form would be similar in that respect. This allows > a limited form of composability between concurrency constructs that is > otherwise hard to achieve. > > For the reasons I’ve already given, I think it would be more useful to > offer higher-level concurrency primitives like events, mutexes, etc., since > those could offer more structure based on the particular use case in > question. (Also, I realized Haskell’s MVars are basically just Racket > channels, though Racket’s channels don’t have a peek operation.) > > More generally, I think Haskell’s concurrency libraries are good prior art > here that would be worth looking at. Haskell’s “asynchronous exceptions” > are directly analogous to Racket’s breaks, though Haskell allows arbitrary > exceptions to be raised asynchronously rather than only allowing the more > restrictive interface of `thread-break`. Haskell’s `mask` operator > correspond’s to Racket’s `parameterize-break`. Even though the primitives > are essentially the same, Haskell’s libraries provide a much richer set of > higher-level abstractions, both in the standard library (see > Control.Exception and Control.Concurrent.*) and in other packages. > > On Jan 18, 2020, at 04:04, Jack Firth <jackhfi...@gmail.com> wrote: > > I am making a new concurrency abstraction, and I already have to work > around the interface because it forces me to make this choice at every use > site. What I was planning on doing was pushing this decision into the value > itself, rather than the use site. So what if `make-semaphore` had a > `#:break-handling-mode` argument that controlled whether or not waiting on > that particular semaphore would either enable breaks, or check that breaks > or disabled, or neither of those? > > On Sat, Jan 18, 2020 at 1:45 AM Alexis King <lexi.lam...@gmail.com> wrote: > >> No, I don’t think so, and here’s why: imagine a library provides an >> abstraction that internally uses semaphores as events. The library uses >> `semaphore-wait` to wait on the event. The client of this library now has >> the option to disable breaks if it turns out this code is actually going to >> be used *inside* a larger critical section, and they don’t want breaks >> to be re-enabled by the library! They really want everything in the >> critical section to keep breaks disabled. So in that case, the >> break-agnostic behavior of `semaphore-wait` really is the right one. >> >> This is what I mean by semaphore’s being a low-level primitive, though. >> There are lots of different behaviors one might want that could be better >> served by higher-level abstractions that can make more assumptions about >> how they’ll be used, but semaphores have to support all of them. I think it >> makes sense that they provide the minimal set of behaviors needed to >> implement those things—it keeps the building blocks as simple and modular >> as possible. You can always implement the more complex behavior on top, but >> it’d be annoying to discover you needed to work around the interface trying >> to protect you from yourself while you’re implementing a new concurrency >> abstraction. >> >> On Jan 18, 2020, at 03:36, Jack Firth <jackhfi...@gmail.com> wrote: >> >> Wouldn't you want to *force* the first thread to wait with >> semaphore-wait/enable-break in that case? Since if they're disabled then >> that thread can't be cooperatively terminated. If you use `semaphore-wait` >> it seems like you completely hand off control over whether breaks are >> enabled or not, which seems like something that use sites should care about >> one way or the other. What sort of semaphore-based communication would be >> truly indifferent to whether breaking is enabled? >> >> On Sat, Jan 18, 2020 at 1:28 AM Alexis King <lexi.lam...@gmail.com> >> wrote: >> >>> Actually, I change my mind, I can trivially think of a case where it’s >>> fine: if you’re just using a semaphore as an event. One thread waits with >>> `semaphore-wait`, another thread calls `semaphore-post`, and after the >>> count is decremented, it’s never re-incremented. It’s just used to gate >>> execution, not guard access to a resource. No need to disable breaks here. >>> >>> (Also, an aside: I think your `car`/`cdr` example is different, because >>> `car`/`cdr`’s checks on pairs guard against memory corruption in the Racket >>> runtime, and Racket is a memory-safe language. A better comparison would be >>> that `car`/`cdr` don’t check whether or not their argument is a proper >>> list—the higher-level `first`/`rest` do that, instead.) >>> >>> On Jan 18, 2020, at 03:21, Jack Firth <jackhfi...@gmail.com> wrote: >>> >>> It isn't clear to me either. I can't think of a use case for it, but I'm >>> hoping either somebody else can or somebody can confirm that it's not a >>> good API precedent. I'm trying to build some concurrency libraries >>> <https://github.com/jackfirth/rebellion/issues/397> and I'd like to be >>> sure there isn't some important use case I'm missing. >>> >>> On Sat, Jan 18, 2020 at 1:14 AM Alexis King <lexi.lam...@gmail.com> >>> wrote: >>> >>>> Like I said, it isn’t clear to me that *all* uses of `semaphore-wait` >>>> when breaks are enabled are incorrect. You could argue that then you should >>>> have a `semaphore-wait/trust-me-even-though-breaks-are-enabled`, and sure, >>>> I don’t think that would necessarily be bad. I just imagine the API just >>>> wasn’t originally designed that way for some reason or another, possibly >>>> simply because it wasn’t considered at the time. Maybe Matthew can give a >>>> more satisfying answer, but I don’t know; I’m just speculating. >>>> >>>> On Jan 18, 2020, at 03:10, Jack Firth <jackhfi...@gmail.com> wrote: >>>> >>>> I don't see how it has to do with semaphores being low-level. If >>>> waiting on a semaphore while breaks are enabled is almost certainly wrong, >>>> checking whether breaks are enabled and raising an error seems like a way >>>> more sensible default behavior than just silently doing something that's >>>> almost certainly wrong. If car and cdr can check their arguments by >>>> default, shouldn't semaphores guard against misuse too? >>>> >>>> On Sat, Jan 18, 2020 at 1:04 AM Alexis King <lexi.lam...@gmail.com> >>>> wrote: >>>> >>>>> It *is *guaranteed to leave the semaphore in a consistent state, from >>>>> the perspective of the implementation of semaphores. No matter what you >>>>> do, >>>>> you won’t ever corrupt a semaphore (assuming you’re not using unsafe >>>>> operations and assuming the runtime is not buggy). >>>>> >>>>> But perhaps you mean inconsistent from the point of view of the >>>>> application, not from the point of view of the Racket runtime. In that >>>>> case, it’s true that when using semaphores as locks, using them in a >>>>> context where breaks are enabled is almost certainly wrong. It’s not >>>>> immediately clear to me that there aren’t any valid uses of semaphores >>>>> where you would want breaks to be enabled, but I admit, I have no idea >>>>> what >>>>> they are. >>>>> >>>>> Semaphores are low-level primitives, though, so I think it makes some >>>>> sense for them to just do the minimal possible thing. Perhaps a library >>>>> ought to offer a slightly more specialized “critical section” abstraction >>>>> a >>>>> la Windows (or perhaps something like Haskell’s MVars) that manages >>>>> disabling interrupts in the critical section for you. (Why doesn’t this >>>>> exist already? My guess is that most Racket programmers don’t worry about >>>>> these details, since they don’t call `break-thread` anywhere, and they >>>>> want >>>>> SIGINT to just kill their process, anyway.) >>>>> >>>>> On Jan 18, 2020, at 02:54, Jack Firth <jackhfi...@gmail.com> wrote: >>>>> >>>>> I do understand all of that, and you're right that "kill-safe" isn't >>>>> what I meant. >>>>> >>>>> What I'm confused about is why, if it's inherently not guaranteed to >>>>> leave the semaphore in a consistent state, semaphore-wait attempts to work >>>>> at all if breaks are enabled. Why not raise some helpful error like "it's >>>>> unsafe to wait on a semaphore while breaks are enabled, did you forget to >>>>> disable breaks?". What's the actual *use case* for calling >>>>> semaphore-wait (and *not* semaphore-wait/enable-break) while breaks >>>>> are enabled? >>>>> >>>>> On Sat, Jan 18, 2020 at 12:47 AM Alexis King <lexi.lam...@gmail.com> >>>>> wrote: >>>>> >>>>>> Killing a thread is different from breaking a thread. Killing a >>>>>> thread kills the thread unrecoverably, and no cleanup actions are run. >>>>>> This >>>>>> usually isn’t what you want, but there’s always a tension between these >>>>>> kinds of things: defensive programmers ask “How do I make myself >>>>>> unkillable >>>>>> so I can safely clean up?” but then implementors of a dynamic environment >>>>>> (like, say, DrRacket) find themselves asking “How do I kill a runaway >>>>>> thread?” Assuming you’re not DrRacket, you usually want `break-thread`, >>>>>> not >>>>>> `kill-thread`. >>>>>> >>>>>> But perhaps you know that already, and your question is just about >>>>>> breaking, so by “kill-safe” you mean “break-safe.” You ask why >>>>>> `semaphore-break` doesn’t just disable breaking, but that wouldn’t help >>>>>> with the problem the documentation alludes to. The problem is that >>>>>> there’s >>>>>> fundamentally a race condition in code like this: >>>>>> >>>>>> (semaphore-wait sem) >>>>>> ; do something important >>>>>> (semaphore-post sem) >>>>>> >>>>>> If this code is executed in a context where breaks are enabled, it’s >>>>>> not break-safe whether or not `semaphore-wait` were to disable breaks >>>>>> while >>>>>> waiting on the semaphore. As soon as `semaphore-wait` returns, the queued >>>>>> break would be delivered, the stack would unwind, and the matching >>>>>> `semaphore-post` call would never execute, potentially holding a lock >>>>>> forever. So the issue isn’t that the semaphore’s internal state gets >>>>>> somehow corrupted, but that the state no longer reflects the value you >>>>>> want. >>>>>> >>>>>> The right way to write that code is to disable breaks in the critical >>>>>> section: >>>>>> >>>>>> (parameterize-break #f >>>>>> (semaphore-wait sem) >>>>>> ; do something important >>>>>> (semaphore-post sem)) >>>>>> >>>>>> This eliminates the race condition, since a break cannot be delivered >>>>>> until the `semaphore-post` executes (and synchronous, non-break >>>>>> exceptions >>>>>> can be protected against via `dynamic-wind` or an exception handler). But >>>>>> this creates a new problem, since if a break is delivered while the code >>>>>> is >>>>>> blocked on the semaphore, it won’t be delivered until the semaphore is >>>>>> posted/unlocked, which may be a very long time. You’d really rather just >>>>>> break the thread, since it hasn’t entered the critical section yet, >>>>>> anyway. >>>>>> >>>>>> This is what `semaphore-wait/enable-break` is for. You can think of >>>>>> it as a version of `semaphore-wait` that re-enables breaks internally, >>>>>> inside its implementation, and it installs an exception handler to ensure >>>>>> that if a break is delivered at the worst possible moment (after the >>>>>> count >>>>>> has been decremented but before breaks are disabled again), it reverses >>>>>> the >>>>>> change and re-raises the break exception. (I have no idea if this is how >>>>>> it’s actually implemented, but I think it’s an accurate model of its >>>>>> behavior.) This does exactly what we want, since it ensures that if we do >>>>>> enter the critical section, breaks are disabled until we exit it, but we >>>>>> can still be interrupted if we’re blocked waiting to enter it. >>>>>> >>>>>> So it’s not so much that there’s anything really special going on >>>>>> here, but more that break safety is inherently anti-modular where state >>>>>> is >>>>>> involved, and you can’t implement `semaphore-wait/enable-break`-like >>>>>> constructs if you only have access to the `semaphore-wait`-like sibling. >>>>>> >>>>>> > On Jan 17, 2020, at 22:37, Jack Firth <jackhfi...@gmail.com> wrote: >>>>>> > >>>>>> > The docs for semaphores say this: >>>>>> > >>>>>> > In general, it is impossible using only semaphore-wait to implement >>>>>> the guarantee that either the semaphore is decremented or an exception is >>>>>> raised, but not both. Racket therefore supplies >>>>>> semaphore-wait/enable-break >>>>>> (see Semaphores), which does permit the implementation of such an >>>>>> exclusive >>>>>> guarantee. >>>>>> > >>>>>> > I understand the purpose of semaphore-wait/enable-break, but >>>>>> there's something about semaphore-wait that confuses me: why does it >>>>>> allow >>>>>> breaking at all? My understanding is that if breaks are enabled, >>>>>> semaphore-wait still tries to block and decrement the counter, even >>>>>> though >>>>>> a break at any time could destroy the integrity of the semaphore. Does >>>>>> that >>>>>> mean it's not kill-safe to use a semaphore as a lock? Wouldn't it be >>>>>> safer >>>>>> if semaphore-wait automatically disabled breaks while waiting? >>>>>> >>>>>> >>>>> >>>> >>> >> > > -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/CAAXAoJXXMjh0SY2Mh8HT%3DfgW4wjaOecRat5M4cFQ2s2dYWa0KQ%40mail.gmail.com.
