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Windows TLS: ManuallyDrop instead of mem::forget #79893

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Dec 11, 2020
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Windows TLS: ManuallyDrop instead of mem::forget #79893

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11 changes: 4 additions & 7 deletions library/std/src/sys/windows/thread_local_key.rs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
use crate::mem;
use crate::mem::ManuallyDrop;
use crate::ptr;
use crate::sync::atomic::AtomicPtr;
use crate::sync::atomic::Ordering::SeqCst;
Expand Down Expand Up @@ -111,16 +111,13 @@ struct Node {
}

unsafe fn register_dtor(key: Key, dtor: Dtor) {
let mut node = Box::new(Node { key, dtor, next: ptr::null_mut() });
let mut node = ManuallyDrop::new(Box::new(Node { key, dtor, next: ptr::null_mut() }));
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Since we never drop this anyway, couldn't we also just Box::into_raw here and then compare_exchangeing the raw pointer directly? I guess that would mean node.next = head needs to be (*node).next = head and is now unsafe, so maybe the ManuallyDrop is better, but I am somewhat unhappy with the &mut **node (the previous &mut *node was not better imo). I know we can rely on the deref ops on boxes, but it still seems wrong to me to not be using Box::into_raw when this is technically what we want.

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I guess that would mean node.next = head needs to be (*node).next = head and is now unsafe

I went with into_raw first and decided to use ManuallyDrop when I realized this.


let mut head = DTORS.load(SeqCst);
loop {
node.next = head;
match DTORS.compare_exchange(head, &mut *node, SeqCst, SeqCst) {
Ok(_) => {
mem::forget(node);
return;
}
match DTORS.compare_exchange(head, &mut **node, SeqCst, SeqCst) {
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This isn't really related, but shouldn't we use compare_exchange_weak here ?

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Possible; I don't really have a clear idea for what should be used when.
Since it's not related, that should then probably be fixed by someone else in a separate PR. :)

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Fine, I'll do it when this PR is merged.

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@a1phyr Did this end up happening?

Ok(_) => return, // nothing to drop, we successfully added the node to the list
Err(cur) => head = cur,
}
}
Expand Down