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fix(subscriber): mitigate race in Callsites::contains #474

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fix(subscriber): mitigate race in Callsites::contains #474

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78483b4
fix(subscriber): mitigate race in `Callsites::contains`
hds Oct 11, 2023
988daeb
Merge branch 'main' into hds/callsites-race
hds Oct 19, 2023
e3c1b56
Merge branch 'main' into hds/callsites-race
hds Oct 25, 2023
8111c70
Merge branch 'main' into hds/callsites-race
hds Oct 25, 2023
5ba6eaf
Merge branch 'main' into hds/callsites-race
hds Oct 25, 2023
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7 changes: 6 additions & 1 deletion console-subscriber/src/callsites.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -46,8 +46,13 @@ impl<const MAX_CALLSITES: usize> Callsites<MAX_CALLSITES> {
let mut len = self.len.load(Ordering::Acquire);
loop {
for cs in &self.ptrs[start..len] {
if ptr::eq(cs.load(Ordering::Acquire), callsite) {
let recorded = cs.load(Ordering::Acquire);
if ptr::eq(recorded, callsite) {
return true;
} else if ptr::eq(recorded, ptr::null_mut()) {
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style nit, take it or leave it: this could be

Suggested change
} else if ptr::eq(recorded, ptr::null_mut()) {
} else if recorded.is_null() {

// We have read a recorded callsite before it has been
// written. We need to check again.
continue;
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hmm, this restarts the whole loop over again. should we, instead, have an inner loop for loading the specific array index until it's no longer null?

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@hawkw After some thinking (and then forgetting about this), I beleive that this change doesn't make sense.

In fact, I'm not sure that the retry mechanism makes sense in general. If tracing gives the guarantee that Subscriber::register_callsite will be called (and finish!) before Subscriber::event get's called, then the retry shouldn't be necessary at all.

If it doesn't give that guarantee, then the retry is increasing our chances of finding the callsite we are interested in during a race condition, but I don't think that it actually solves the problem either.

For now I'm going to close this PR. After looking at tokio-rs/tracing#2743 a bit more I'll revisit.

}
}

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