shim Apple's futex primitives #4142
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Thanks for the PR! I just left for a 1-week business trip and will have to catch up with stuff when I am back so it could take a bit before I can take a closer look. |
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Jan 29, 2025
| fn wake_nobody() { | ||
| let futex = 0; | ||
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| // Wake 1 waiter. Expect zero waiters woken up, as nobody is waiting. |
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Suggested change
| // Wake 1 waiter. Expect zero waiters woken up, as nobody is waiting. | |
| // Wake 1 waiter. Expect ENOENT, as nobody is waiting. |
| size_of::<i32>(), | ||
| libc::OS_SYNC_WAIT_ON_ADDRESS_NONE, | ||
| ), | ||
| 0, |
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Please add a test where wait_on_address returns something non-zero (since there are more waiters).
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While doing this, I noticed that the reported count was incorrect if os_sync_wake_by_address_all was used as the threads were woken one-by-one. dfa8d2fa13d2c9972208ab7dbc62e197a44f1920 fixes this (and also gets rid of the quadratic runtime of wakeup 😄).
This is necessary to unblock rust-lang/rust#122408. The documentation for these is available [here](https://developer.apple.com/documentation/os/os_sync_wait_on_address?language=objc). Because the futex wait operations (`os_sync_wait_on_address` et al.) return the number of remaining waiters after returning, this required some changes to the common futex infrastructure, which I've changed to take a callback instead of precalculating the return values.
On macOS, the futex operations return the number of outstanding waiters. For this count to be correct, we have to remove all eligible threads from the internal futex list before any of the wakeup callbacks are run. This has the nice side-effect of making the runtime removal operation linear in the number of threads instead of quadratic.
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This is necessary to unblock rust-lang/rust#122408. The documentation for these is available here.
Because the futex wait operations (
os_sync_wait_on_addresset al.) return the number of remaining waiters after returning, this required some changes to the common futex infrastructure, which I've changed to take a callback instead of precalculating the return values.