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Re: [std-proposals] Fwd: set_new_handler extension

From: Phil Bouchard <boost_at_[hidden]>
Date: Thu, 1 Jun 2023 11:04:26 -0400
On 6/1/23 10:42, Jason McKesson via Std-Proposals wrote:

>>> Are you suggesting that the compiler analyse the if-find line, find all lockable
>>> elements in use there, then lock them behind the scenes without user action
>>> and keep them locked until the end of the scope?
>> Yes that's what I am trying to say since the beginning.
> That is not possible in general. Some of that stuff can be hidden
> behind function calls that are opaque to the compiler. It does not
> know what gets called in that code, and since the object is shared, it
> may not even be obvious whether the opaque function call can access
> it.
> Therefore, the compiler has two choices: don't automagically engage
> the lock (and therefore potentially break code), or do automagically
> engage the lock. But the latter can be pretty bad too; if the opaque
> function doesn't use it, you now lock the container for an
> indeterminate period of time. Especially since the opaque function can
> have arbitrary amounts of code in it.

Why would it be indeterminate assuming you have a
"thread-scope-recursive" mutex?

>> Here is just an
>> example of the net code the compiler would see after making temporary
>> variables last for the duration of the conditional scope:
>> https://github.com/philippeb8/std__ts/blob/master/ts.cpp#L23
>>>> Again, where's the problem if both containers feature a recursive mutex?
>>> Recursion is not the problem. Atomicity of the action is. The point I tried to
>>> make with the previous example is that one must reason about the duration of
>>> the lock to decide when it must start and when it must end, so the conditions
>>> don't change between statements. This is also the analogy of the transaction
>>> that someone else posted in this thread.
>>> With the simple examples we're discussing here, it might be obvious what to do
>>> and thus make solutions obvious. What others and I are telling you is that
>>> when it gets to really complex thread-safe code, you *have* to reason about
>>> when locks must start and when they must end, and what other locks you have.
>>> Plus, reason about the order of locks, to avoid deadlocks.
>>> Thread-safety requires having the smallest possible critical sections, but no
>>> smaller. If you pulverise your locks, you add overhead and actually lose
>>> safety.
>>>> Regarding teaching, this is higher-level programming so a new namespace
>>>> should encompass these new classes.
>>> That's not what I meant. You're oversimplifying the answer to a complex
>>> question. Refer back to the top of this email:
>>> if (!container.empty())
>>> container.push)back(1);
>>> This may have no data race and thus cause no data corruption, but it's not
>>> what was required because the states may have changed. If this is still
>>> allowed, then just using the container in question does *not* confer thread-
>>> safety. And therefore, if it is allowed to compile, how do you propose we
>>> teach everyone *how* to write code to actually make it thread-safe?
>> You add some type trait allowing the compiler to determine whether the
>> class is a "thread-safe" class or not.
> This is what I like to call "bulldozer design". You declare that you
> want X, and when people point out problems with achieving X, you take
> a bulldozer to whatever those problems are. Every time someone points
> out a problem, you say "oh, we'll add a niche solution to fix that."

What you call "bulldozer design" I call "intelligently abstracted

Given you guys are the smartest C++ developers in the world, I would
have expected a more positive feedback.

> You want a thread-safe container. Such containers have substantial
> flaws that make using them in a thread-safe way difficult and
> perilous. Therefore, every time someone shows such a circumstance, you
> try to come up with some small language change to allow compilers to
> fix it.
> This kind of design often creates a bunch of tiny-yet-complex language
> features that individually don't have any real meaning.
> It's also a bad way to suggest a feature for standardization. If you
> want to bulldozer your way to an idea, you need to figure out what the
> problems are *yourself* and present a fully bulldozed path from here
> to there. It's not our job to point out all of the houses and
> buildings in the path of your bulldozer.
>>> And how is that different from what we're already doing now?
>> BTW forget my previous Github example, it is not generic enough. A
>> thread-safe smart pointer (root_ptr or atomic<shared_ptr>) or the
>> following wrapper with some type trait would be the way to go:
>> https://fekir.info/post/sharing-data-between-threads/#_bind-the-data-and-mutex-together
>> So for each temporary variable being a "thread_safe_type" object, the
>> compiler would generate temporary variables lasting for the scope of the
>> condition.
>> But again we would need also new "thread-recursive" on top of
>> "scope-recursive" mutices.
> And how do you fix this:
> ```
> container.push_back(1);
> container.push_back(2);
> ```

Again that's the algorithmic level the compiler wouldn't interfere with.

> Did the user expect these two operations to be atomic or not? That is,
> does the user expect the outside world to observe these changes
> individually or as a collective? You don't know and you *cannot* know.
> These are distinct things and must be explicitly written distinctly.
> Which means that users still have to pay attention when they share
> containers.
> The idea that you can write asynchronous code exactly like synchronous
> code and the compiler will come along after you to fix all of your
> problems is folly. It ain't happening. Not unless you do it in a
> language and context that is expressly designed for that.

I'm not saying to forcibly change all containers but you simply add an
optional generic wrapper that can be used.

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Received on 2023-06-01 15:04:27