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Tracking issue for cleaning up std's thread_local implementation details #110897
Comments
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@joboet In what situation would that |
It's very cursed, but this works. I don't think it's currently tested, however. |
That just seems like a bug to me. A (thread local) |
Maybe 😄? It's sound however, as there can be no references to the data from the first initialization when the new value is written. |
Sure, but so would OnceCell be. ^^ And I think users should be able to safely assume that an immutable |
I did some software archeology and found the original issue and PR that introduced that mem::replace call: #30228 and #30267. The issue was originally about thread locals, but then most of the discussion was about drop on assignment in general. The issue description suggests:
Without a preference for any of these solutions. The PR also doesn't include a reason for picking one solution over the other. So I don't think the mem::replace behaviour was purposely picked over another solution (like dropping the other value or panicking). |
…-key, r=cuviper Remove unused std::sys_common::thread_local_key::Key Part of rust-lang#110897 This `Key` type seems unused. Let's remove it and see if anything explodes. :)
In this case I don't think anyone actually relies on LazyKeyInner being a OnceOrMaybeTwiceIfYouTryReallyHardCell instead of just a OnceCell. ^^ |
I'd advocate for a crater run, just in case, but yes. |
It is extremely unlikely someone is going to have tests that stress this exact point sufficiently for it to be revealed. |
Ok, yes, it's actually part of the documentation. Nevermind, then. |
Thanks for this, I've been very unhappy with the
There are a few others I had written down (multiple implementations of the manual tls-dtor list) but you touch on them and it might be outdated given #109858. |
…uviper Remove unused std::sys_common::thread_local_key::Key Part of rust-lang/rust#110897 This `Key` type seems unused. Let's remove it and see if anything explodes. :)
…n, r=dtolnay Broaden the consequences of recursive TLS initialization This PR updates the documentation of `LocalKey` to clearly disallow the behaviour described in [this comment](rust-lang#110897 (comment)). This allows using `OnceCell` for the lazy initialization of TLS variables, which panics on reentrant initialization instead of updating the value like TLS variables currently do. `@rustbot` label +T-libs-api r? `@m-ou-se`
…n, r=dtolnay Broaden the consequences of recursive TLS initialization This PR updates the documentation of `LocalKey` to clearly disallow the behaviour described in [this comment](rust-lang#110897 (comment)). This allows using `OnceCell` for the lazy initialization of TLS variables, which panics on reentrant initialization instead of updating the value like TLS variables currently do. ``@rustbot`` label +T-libs-api r? ``@m-ou-se``
Rollup merge of rust-lang#116172 - joboet:recursive_tls_initialization, r=dtolnay Broaden the consequences of recursive TLS initialization This PR updates the documentation of `LocalKey` to clearly disallow the behaviour described in [this comment](rust-lang#110897 (comment)). This allows using `OnceCell` for the lazy initialization of TLS variables, which panics on reentrant initialization instead of updating the value like TLS variables currently do. ``@rustbot`` label +T-libs-api r? ``@m-ou-se``
Rewrite native thread-local storage (part of rust-lang#110897) The current native thread-local storage implementation has become quite messy, uses indescriptive names and unnecessarily adds code to the macro expansion. This PR tries to fix that by using a new implementation that also allows more layout optimizations and potentially increases performance by eliminating unnecessary TLS accesses. This does not change the recursive initialization behaviour I described in [this comment](rust-lang#110897 (comment)), so it should be a library-only change. Changing that behaviour should be quite easy now, however. r? `@m-ou-se` `@rustbot` label +T-libs
Rewrite native thread-local storage (part of rust-lang#110897) The current native thread-local storage implementation has become quite messy, uses indescriptive names and unnecessarily adds code to the macro expansion. This PR tries to fix that by using a new implementation that also allows more layout optimizations and potentially increases performance by eliminating unnecessary TLS accesses. This does not change the recursive initialization behaviour I described in [this comment](rust-lang#110897 (comment)), so it should be a library-only change. Changing that behaviour should be quite easy now, however. r? `@m-ou-se` `@rustbot` label +T-libs
Rewrite native thread-local storage (part of rust-lang#110897) The current native thread-local storage implementation has become quite messy, uses indescriptive names and unnecessarily adds code to the macro expansion. This PR tries to fix that by using a new implementation that also allows more layout optimizations and potentially increases performance by eliminating unnecessary TLS accesses. This does not change the recursive initialization behaviour I described in [this comment](rust-lang#110897 (comment)), so it should be a library-only change. Changing that behaviour should be quite easy now, however. r? `@m-ou-se` `@rustbot` label +T-libs
Simplify key-based thread locals This PR simplifies key-based thread-locals by: * unifying the macro expansion of `const` and non-`const` initializers * reducing the amount of code in the expansion * simply reallocating on recursive initialization instead of going through `LazyKeyInner` * replacing `catch_unwind` with the shared `abort_on_dtor_unwind` It does not change the initialization behaviour described in rust-lang#110897.
Rewrite native thread-local storage (part of rust-lang#110897) The current native thread-local storage implementation has become quite messy, uses indescriptive names and unnecessarily adds code to the macro expansion. This PR tries to fix that by using a new implementation that also allows more layout optimizations and potentially increases performance by eliminating unnecessary TLS accesses. This does not change the recursive initialization behaviour I described in [this comment](rust-lang#110897 (comment)), so it should be a library-only change. Changing that behaviour should be quite easy now, however. r? `@m-ou-se` `@rustbot` label +T-libs
Rewrite TLS on platforms without threads The saga of rust-lang#110897 continues! r? `@m-ou-se` if you have time
Rewrite native thread-local storage (part of #110897) The current native thread-local storage implementation has become quite messy, uses indescriptive names and unnecessarily adds code to the macro expansion. This PR tries to fix that by using a new implementation that also allows more layout optimizations and potentially increases performance by eliminating unnecessary TLS accesses. This does not change the recursive initialization behaviour I described in [this comment](rust-lang/rust#110897 (comment)), so it should be a library-only change. Changing that behaviour should be quite easy now, however. r? `@m-ou-se` `@rustbot` label +T-libs
Simplify key-based thread locals This PR simplifies key-based thread-locals by: * unifying the macro expansion of `const` and non-`const` initializers * reducing the amount of code in the expansion * simply reallocating on recursive initialization instead of going through `LazyKeyInner` * replacing `catch_unwind` with the shared `abort_on_dtor_unwind` It does not change the initialization behaviour described in rust-lang#110897.
Rewrite native thread-local storage (part of #110897) The current native thread-local storage implementation has become quite messy, uses indescriptive names and unnecessarily adds code to the macro expansion. This PR tries to fix that by using a new implementation that also allows more layout optimizations and potentially increases performance by eliminating unnecessary TLS accesses. This does not change the recursive initialization behaviour I described in [this comment](rust-lang/rust#110897 (comment)), so it should be a library-only change. Changing that behaviour should be quite easy now, however. r? `@m-ou-se` `@rustbot` label +T-libs
As discovered by Mara in rust-lang#110897, our TLS implementation is a total mess. In the past months, I have simplified the actual macros and their expansions, but the majority of the complexity comes from the platform-specific support code needed to create keys and register destructors. In keeping with rust-lang#117276, I have therefore moved all of the `thread_local_key`/`thread_local_dtor` functions to the `thread_local` module in `sys` and given them a new structure, so that future porters of `std` can simply mix-and-match the existing code instead of having to copy the same (bad) implementation everywhere. The new structure should become obvious when looking at `sys/thread_local/mod.rs`. Unfortunately, the documentation changes associated with the refactoring have made this PR rather large. That said, this contains no functional changes except for two small ones: * the key-based destructor fallback now, by virtue of sharing the implementation used by macOS and others, stores its list in a `#[thread_local]` static instead of in the key, eliminating one indirection layer and drastically simplifying its code. * I've switched over ZKVM (tier 3) to use the same implementation as WebAssembly, as the implementation was just a way worse version of that Please let me know if I can make this easier to review! I know these large PRs aren't optimal, but I couldn't think of any good intermediate steps. @rustbot label +A-thread-locals
As discovered by Mara in rust-lang#110897, our TLS implementation is a total mess. In the past months, I have simplified the actual macros and their expansions, but the majority of the complexity comes from the platform-specific support code needed to create keys and register destructors. In keeping with rust-lang#117276, I have therefore moved all of the `thread_local_key`/`thread_local_dtor` modules to the `thread_local` module in `sys` and merged them into a new structure, so that future porters of `std` can simply mix-and-match the existing code instead of having to copy the same (bad) implementation everywhere. The new structure should become obvious when looking at `sys/thread_local/mod.rs`. Unfortunately, the documentation changes associated with the refactoring have made this PR rather large. That said, this contains no functional changes except for two small ones: * the key-based destructor fallback now, by virtue of sharing the implementation used by macOS and others, stores its list in a `#[thread_local]` static instead of in the key, eliminating one indirection layer and drastically simplifying its code. * I've switched over ZKVM (tier 3) to use the same implementation as WebAssembly, as the implementation was just a way worse version of that Please let me know if I can make this easier to review! I know these large PRs aren't optimal, but I couldn't think of any good intermediate steps. @rustbot label +A-thread-locals
… r=Mark-Simulacrum std: refactor the TLS implementation As discovered by Mara in rust-lang#110897, our TLS implementation is a total mess. In the past months, I have simplified the actual macros and their expansions, but the majority of the complexity comes from the platform-specific support code needed to create keys and register destructors. In keeping with rust-lang#117276, I have therefore moved all of the `thread_local_key`/`thread_local_dtor` modules to the `thread_local` module in `sys` and merged them into a new structure, so that future porters of `std` can simply mix-and-match the existing code instead of having to copy the same (bad) implementation everywhere. The new structure should become obvious when looking at `sys/thread_local/mod.rs`. Unfortunately, the documentation changes associated with the refactoring have made this PR rather large. That said, this contains no functional changes except for two small ones: * the key-based destructor fallback now, by virtue of sharing the implementation used by macOS and others, stores its list in a `#[thread_local]` static instead of in the key, eliminating one indirection layer and drastically simplifying its code. * I've switched over ZKVM (tier 3) to use the same implementation as WebAssembly, as the implementation was just a way worse version of that Please let me know if I can make this easier to review! I know these large PRs aren't optimal, but I couldn't think of any good intermediate steps. `@rustbot` label +A-thread-locals
…=Mark-Simulacrum std: refactor the TLS implementation As discovered by Mara in rust-lang#110897, our TLS implementation is a total mess. In the past months, I have simplified the actual macros and their expansions, but the majority of the complexity comes from the platform-specific support code needed to create keys and register destructors. In keeping with rust-lang#117276, I have therefore moved all of the `thread_local_key`/`thread_local_dtor` modules to the `thread_local` module in `sys` and merged them into a new structure, so that future porters of `std` can simply mix-and-match the existing code instead of having to copy the same (bad) implementation everywhere. The new structure should become obvious when looking at `sys/thread_local/mod.rs`. Unfortunately, the documentation changes associated with the refactoring have made this PR rather large. That said, this contains no functional changes except for two small ones: * the key-based destructor fallback now, by virtue of sharing the implementation used by macOS and others, stores its list in a `#[thread_local]` static instead of in the key, eliminating one indirection layer and drastically simplifying its code. * I've switched over ZKVM (tier 3) to use the same implementation as WebAssembly, as the implementation was just a way worse version of that Please let me know if I can make this easier to review! I know these large PRs aren't optimal, but I couldn't think of any good intermediate steps. `@rustbot` label +A-thread-locals
…=Mark-Simulacrum std: refactor the TLS implementation As discovered by Mara in rust-lang#110897, our TLS implementation is a total mess. In the past months, I have simplified the actual macros and their expansions, but the majority of the complexity comes from the platform-specific support code needed to create keys and register destructors. In keeping with rust-lang#117276, I have therefore moved all of the `thread_local_key`/`thread_local_dtor` modules to the `thread_local` module in `sys` and merged them into a new structure, so that future porters of `std` can simply mix-and-match the existing code instead of having to copy the same (bad) implementation everywhere. The new structure should become obvious when looking at `sys/thread_local/mod.rs`. Unfortunately, the documentation changes associated with the refactoring have made this PR rather large. That said, this contains no functional changes except for two small ones: * the key-based destructor fallback now, by virtue of sharing the implementation used by macOS and others, stores its list in a `#[thread_local]` static instead of in the key, eliminating one indirection layer and drastically simplifying its code. * I've switched over ZKVM (tier 3) to use the same implementation as WebAssembly, as the implementation was just a way worse version of that Please let me know if I can make this easier to review! I know these large PRs aren't optimal, but I couldn't think of any good intermediate steps. `@rustbot` label +A-thread-locals
This might have caused #126992 |
std: separate TLS key creation from TLS access Currently, `std` performs an atomic load to get the OS key on every access to `StaticKey` even when the key is already known. This PR thus replaces `StaticKey` with the platform-specific `get` and `set` function and a new `LazyKey` type that acts as a `LazyLock<Key>`, allowing the reuse of the retreived key for multiple accesses. Related to rust-lang#110897.
Rollup merge of rust-lang#126953 - joboet:lazy_key, r=jhpratt std: separate TLS key creation from TLS access Currently, `std` performs an atomic load to get the OS key on every access to `StaticKey` even when the key is already known. This PR thus replaces `StaticKey` with the platform-specific `get` and `set` function and a new `LazyKey` type that acts as a `LazyLock<Key>`, allowing the reuse of the retreived key for multiple accesses. Related to rust-lang#110897.
std::thread_local
,std::thread::local
,std::thread::local_impl
,std::sys_common::thread_local_key
,std::sys_common::thread_local_dtor
,std::sys::thread_local_key
, etc. etc. are all messy and form quite a confusing maze. Just look at this map I tried to draw of it all:AAAaaaaAaa
Time to clean it up.
And maybe also fix some bugs while we're at it. ^^
Key
type: Remove unused std::sys_common::thread_local_key::Key #110898extern "C"
inthread_local::fast_local
#112292Key
, because most of those things are actually not keys, and dropLazyKeyInner
thread_local!{}
expansion. Make TLS accessors closures that return pointers #125525*const T
rather than&'static T
when it's not really'static
: Make TLS accessors closures that return pointers #125525#[thread_local]
)?ReentrantLock
#124881thread::current
thread local more efficient and make it available in (most) TLS destructors: std: makethread::current
available in allthread_local!
destructors #127912Related issues to solve, maybe:
Stack overflow in thread local's drop rendered as a segmentation fault. #109785The text was updated successfully, but these errors were encountered: