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[SR-12838] Cygwin: _swift_once_f has a race condition #55284

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belkadan opened this issue May 19, 2020 · 0 comments
Open

[SR-12838] Cygwin: _swift_once_f has a race condition #55284

belkadan opened this issue May 19, 2020 · 0 comments

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@belkadan
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@belkadan belkadan commented May 19, 2020

Previous ID SR-12838
Radar None
Original Reporter @belkadan
Type Improvement
Additional Detail from JIRA
Votes 0
Component/s Standard Library
Labels Improvement, Cygwin, Runtime
Assignee None
Priority Medium

md5: 97f5aae70837cdc3a6fe66c2376dd77e

Issue Description:

Cygwin has a custom implementation of swift_once because the compiler only reserves a single word for the "predicate". However, that implementation (in stdlib/public/runtime/CygwinPort.cpp) has a race condition:

  • Thread A calls swift_once

  • Thread B calls swift_once

  • Thread A reads 0 from the predicate

  • Thread A stores 1 into the predicate

  • Thread B reads 1 from the predicate

  • Thread B returns (!!)

  • Thread A executes the initialization

There are really three states here: not-yet-run, running, and already-run.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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