Lazily allocate and initialize pthread locks. #97647

m-ou-se · 2022-06-02T10:38:50Z

Lazily allocate and initialize pthread locks.

This allows {Mutex, Condvar, RwLock}::new() to be const, while still using the platform's native locks for features like priority inheritance and debug tooling. E.g. on macOS, we cannot directly use the (private) APIs that pthread's locks are implemented with, making it impossible for us to use anything other than pthread while still preserving priority inheritance, etc.

This PR doesn't yet make the public APIs const. That's for a separate PR with an FCP.

Tracking issue: #93740

library/std/src/sys_common/condvar/check.rs

m-ou-se · 2022-06-02T12:04:09Z

library/std/src/sys_common/lazy_box.rs

+    _phantom: PhantomData<T>,
+}
+
+pub(crate) unsafe trait LazyInit {


I had to make this trait pub(crate) instead of pub, otherwise rustc will start suggesting to implement std::sys_common::lazy_box::LazyInit every time a user tries to call a non-existent new function. And due to it being a bound on LazyBox, the change from pub to pub(crate) went a bit viral through std::sys.

See #95080

library/std/src/sys_common/lazy_box.rs

library/std/src/sys/sgx/condvar.rs

Amanieu · 2022-06-02T15:18:16Z

library/std/src/sys_common/lazy_box.rs

+    unsafe fn init(&mut self);
+    /// This is called before deallocating the box, with the same address for
+    /// `self` as used in `init()`.
+    unsafe fn destroy(&mut self);


Why is destroy still needed? Can't we just use the Drop impl?

Why is destroy still needed? Can't we just use the Drop impl?

That's a leftover from the very weird interface of sys's Mutex: init() and destroy() are "optional".

Since #77147, however, the only cases where destroy() is not called are for statics, which don't get dropped anyway. So moving this to a Drop impl should work now.

library/std/src/sys_common/lazy_box.rs

nbdd0121 · 2022-06-02T16:47:19Z

How about just making LazyInit have a single function fn new() -> Pin<Box<Self>>?

Amanieu · 2022-06-02T17:00:23Z

How about just making LazyInit have a single function fn new() -> Pin<Box<Self>>?

At that point, why both with a trait? Just have LazyBox::new take an impl FnMut() -> Pin<Box<Self>>.

m-ou-se · 2022-06-03T15:16:04Z

At that point, why both with a trait? Just have LazyBox::new take an impl FnMut() -> Pin<Box<Self>>.

Because that new function is called as MovableMutex::new(), and MovableMutex is either an alias for sys::locks::Mutex, or for LazyBox<sys::locks::Mutex>, depending on the platform. So calling <_>::new() has to work on both.

Amanieu · 2022-06-03T15:51:01Z

@bors r+

Looking forward to finally making Mutex::new const!

bors · 2022-06-03T15:51:02Z

📌 Commit 6a417d4 has been approved by Amanieu

Lazily allocate and initialize pthread locks. Lazily allocate and initialize pthread locks. This allows {Mutex, Condvar, RwLock}::new() to be const, while still using the platform's native locks for features like priority inheritance and debug tooling. E.g. on macOS, we cannot directly use the (private) APIs that pthread's locks are implemented with, making it impossible for us to use anything other than pthread while still preserving priority inheritance, etc. This PR doesn't yet make the public APIs const. That's for a separate PR with an FCP. Tracking issue: rust-lang#93740

Rollup of 5 pull requests Successful merges: - rust-lang#96642 (Avoid zero-sized allocs in ThinBox if T and H are both ZSTs.) - rust-lang#97647 (Lazily allocate and initialize pthread locks.) - rust-lang#97715 (Support the `#[expect]` attribute on fn parameters (RFC-2383)) - rust-lang#97716 (Fix reachability analysis for const methods) - rust-lang#97722 (Tighten spans for bad fields in struct deriving `Copy`) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

Make {Mutex, Condvar, RwLock}::new() const. This makes it possible to have `static M: Mutex<_> = Mutex::new(..);` 🎉 Our implementations [on Linux](rust-lang#95035), [on Windows](rust-lang#77380), and various BSDs and some tier 3 platforms have already been using a non-allocating const-constructible implementation. As of rust-lang#97647, the remaining platforms (most notably macOS) now have a const-constructible implementation as well. This means we can finally make these functions publicly const. Tracking issue: rust-lang#93740

Hermit: Fix initializing lazy locks Closes hermit-os/hermit-rs#322. The initialization function of hermit's `Condvar` is not called since rust-lang#97647 and was erroneously removed in rust-lang#97879. r? `@m-ou-se` CC: `@stlankes`

Hermit: Fix initializing lazy locks Closes hermit-os/hermit-rs#322. The initialization function of hermit's `Condvar` is not called since rust-lang#97647 and was erroneously removed in rust-lang#97879. r? ``@m-ou-se`` CC: ``@stlankes``

Make {Mutex, Condvar, RwLock}::new() const. This makes it possible to have `static M: Mutex<_> = Mutex::new(..);` 🎉 Our implementations [on Linux](rust-lang/rust#95035), [on Windows](rust-lang/rust#77380), and various BSDs and some tier 3 platforms have already been using a non-allocating const-constructible implementation. As of rust-lang/rust#97647, the remaining platforms (most notably macOS) now have a const-constructible implementation as well. This means we can finally make these functions publicly const. Tracking issue: rust-lang/rust#93740

Hermit: Fix initializing lazy locks Closes hermit-os/hermit-rs#322. The initialization function of hermit's `Condvar` is not called since rust-lang/rust#97647 and was erroneously removed in rust-lang/rust#97879. r? ``@m-ou-se`` CC: ``@stlankes``

m-ou-se added A-concurrency Area: Concurrency T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jun 2, 2022

m-ou-se assigned Amanieu Jun 2, 2022

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jun 2, 2022

m-ou-se mentioned this pull request Jun 2, 2022

Tracking issue for improving std::sync::{Mutex, RwLock, Condvar} #93740

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m-ou-se force-pushed the lazy-box-locks branch from 2dad6ee to 40fa7f1 Compare June 2, 2022 12:42

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m-ou-se added 2 commits June 3, 2022 16:45

Use Drop instead of destroy() for locks.

ac5aa1d

Lazily allocate+initialize locks.

6a417d4

m-ou-se force-pushed the lazy-box-locks branch from 40fa7f1 to 6a417d4 Compare June 3, 2022 15:04

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jun 3, 2022

Dylan-DPC mentioned this pull request Jun 4, 2022

Rollup of 5 pull requests #97727

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Dylan-DPC mentioned this pull request Jun 4, 2022

Rollup of 5 pull requests #97729

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bors merged commit e9ec022 into rust-lang:master Jun 4, 2022

rustbot added this to the 1.63.0 milestone Jun 4, 2022

m-ou-se deleted the lazy-box-locks branch June 4, 2022 14:26

m-ou-se mentioned this pull request Jun 6, 2022

Make {Mutex, Condvar, RwLock}::new() const. #97791

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tmandry mentioned this pull request Jun 20, 2022

Use futex based thread parker on Fuchsia. #96768

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This was referenced Jun 26, 2022

Condvar hangs in newer rust versions hermit-os/hermit-rs#322

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Hermit: Fix initializing lazy locks #98555

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Lazily allocate and initialize pthread locks. #97647

Lazily allocate and initialize pthread locks. #97647

m-ou-se commented Jun 2, 2022

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Lazily allocate and initialize pthread locks. #97647

Lazily allocate and initialize pthread locks. #97647

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