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std: use a mutex for ThreadId generation if available - #149476
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rustbot
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Nov 30, 2025
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rustbot has assigned @Mark-Simulacrum. Use |
joboet
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Nov 30, 2025
| #[cold] | ||
| fn exhausted() -> ! { | ||
| panic!("failed to generate unique thread ID: bitspace exhausted") | ||
| rtabort!("failed to generate unique thread ID: bitspace exhausted") |
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Panicking may allocate, so it shouldn't be used here either.
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This violates the contract we established that the global allocator may call So if there's a platform without 64-bit atomics but which has an allocating |
joboet
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Nov 30, 2025
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No it won't, the |
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@joboet Actually, re-reading the code, neither If we were to adopt #147342 I think we might be able to get rid of the desperate path altogether. |
joboet
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Nov 30, 2025
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The problem is that not all threads are spawned by Rust – and on foreign threads, it's the first call to |
orlp
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Nov 30, 2025
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I hadn't considered foreign threads, you're right. |
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RalfJung
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Nov 30, 2025
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This still invokes |
| } | ||
| } | ||
| /// Generates a new unique thread ID. |
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| /// Generates a new unique thread ID. | |
| /// Generates a new unique thread ID. Might use the global allocator. |
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ThreadId generation fallback path: avoid spurious yields Fixesrust-lang/miri#4737 Alternative to rust-lang#149476 Cc `@orlp` `@joboet`
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ThreadId generation fallback path: avoid spurious yields Fixesrust-lang/miri#4737 Alternative to #149476 Cc `@orlp` `@joboet`
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ThreadId generation fallback path: avoid spurious yields Fixesrust-lang/miri#4737 Alternative to #149476 Cc `@orlp` `@joboet`
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ThreadId generation fallback path: avoid spurious yields Fixesrust-lang/miri#4737 Alternative to #149476 Cc `@orlp` `@joboet`
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This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed. Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers. |
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ThreadId generation fallback path: avoid spurious yields Fixes#4737 Alternative to rust-lang/rust#149476 Cc `@orlp` `@joboet`
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Dec 5, 2025
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I don't think this really matters, closing. |
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ThreadId generation fallback path: avoid spurious yields Fixesrust-lang/miri#4737 Alternative to rust-lang#149476 Cc `@orlp` `@joboet`
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Fixesrust-lang/miri#4737
#144465 changed
ThreadId::newto use a spinlock on platforms without 64-bit atomics. This introduces subtle scheduling changes such as the one described in rust-lang/miri#4737 and isn't ideal from a performance perspective either. The necessity of avoidingMutexis not always given however – it only needs to be avoided if the thread handle is lazily initialised. For normal Rust threads however,stdgenerates the thread id inside the spawning logic, where allocation is expected. Thus this PR renames the currentThreadId::newtodesperate_newand reintroduces a versionThreadId::newthat employs aMutexto synchronise access to the inner spin-lock.CC @RalfJung@orlp