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replace parking_lot with parking_lot_core #178

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merged 5 commits into from May 19, 2022
Merged

replace parking_lot with parking_lot_core #178

merged 5 commits into from May 19, 2022

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matklad
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@matklad matklad commented May 18, 2022

This is primaraliy to enable #177 for pl as well

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matklad commented May 18, 2022

@pitdicker if by any chance you have resources and desire to make a review here, that'd be helpful! No worries if you don't!

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@nagisa nagisa left a comment

Did you consider setting up loupe to actually verify the atomics?

}

/// Safety: synchronizes with store to value via Release/Acquire.
#[inline]
pub(crate) fn is_initialized(&self) -> bool {
self.is_initialized.load(Ordering::Acquire)
self.state.load(Ordering::Acquire) == COMPLETE
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@nagisa nagisa May 19, 2022

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This can probably be a plain Relaxed load.

The use of this API can only ever be used in an informational manner (it is 100% racy no matter how the returned value is used…) so it doesn't matter if it spuriously returns false for a little while when the once cell is already initialized.

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@matklad matklad May 19, 2022

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This one I am rather sure should be Acquire, as it guards get_unchecked accesses to the value.

once_cell/src/lib.rs

Lines 864 to 867 in b57c774

pub fn get(&self) -> Option<&T> {
if self.0.is_initialized() {
// Safe b/c value is initialized.
Some(unsafe { self.get_unchecked() })

src/imp_pl.rs Outdated
let key = state as *const _ as usize;
loop {
let exchange =
state.compare_exchange(INCOMPLETE, RUNNING, Ordering::Acquire, Ordering::Acquire);
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@nagisa nagisa May 19, 2022

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Suggested change
state.compare_exchange(INCOMPLETE, RUNNING, Ordering::Acquire, Ordering::Acquire);
state.compare_exchange(INCOMPLETE, RUNNING, Ordering::Acquire, Ordering::Relaxed);

You could also consider compare_exchange_weak, though I don't know if there's any benefit in doing so since you will need to continue on Err(INCOMPLETE) anyway.

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@matklad matklad May 19, 2022

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Hm, this I think also needs to be acquire, otherwise Err(COMPLETE) => return, branch won't synchronize-with the corresponding Release store, and subsequent get_unchecked would read garbage. Both cases are great to check if loom works though!

matklad added 2 commits May 19, 2022
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matklad commented May 19, 2022

Did you consider setting up loupe to actually verify the atomics?

loupe meaning loom? Yeah, I did some manual hacking with loom, and it indeed didn't complain about the current impl, and did complain about relaxed.

Though, pushing that to CI is a bit challenging, as it doesn't support const-fn construction of atomics, something we rely on. Need to think about a nice way to work-around that...

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matklad commented May 19, 2022

bors r+

bors bot added a commit that referenced this issue May 19, 2022
178: replace parking_lot with parking_lot_core r=matklad a=matklad

This is primaraliy to enable #177 for pl as well

Co-authored-by: Aleksey Kladov <aleksey.kladov@gmail.com>
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bors bot commented May 19, 2022

Canceled.

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matklad commented May 19, 2022

bors r+

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bors bot commented May 19, 2022

Build succeeded:

@bors bors bot merged commit 12f283f into master May 19, 2022
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@bors bors bot deleted the plc branch May 19, 2022
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2 participants