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

[belkadan]

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.