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Fix deadlock in InternedIngredient::intern

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This commit is contained in:
Jack Rickard 2022-09-16 00:10:33 +01:00
parent 61e3bdfc81
commit 79823c7f65
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GPG key ID: 88084D7D08A72C8A
1 changed files with 24 additions and 13 deletions
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37
components/salsa-2022/src/interned.rs
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@ -89,20 +89,31 @@ where
return *id;
}
match self.key_map.entry(data.clone()) {
// Data has already been interned, just use the existing ID
dashmap::mapref::entry::Entry::Occupied(entry) => *entry.get(),
// Insert the new value at the next available id
// This holds a lock on the entry so other can't attempt to insert
// at the same time.
let next_id = self.counter.fetch_add(1);
let next_id = Id::from_id(crate::id::Id::from_u32(next_id));
// Insert the data into the value map. This temporarily breaks the
// invariant that value_map and key_map are opposites, this is fine
// because bno-one has the value next_id so it's not observable.
let old_value = self.value_map.insert(next_id, Box::new(data.clone()));
assert!(
old_value.is_none(),
"next_id is guaranteed to be unique, bar overflow"
);
match self.key_map.entry(data) {
// Data has been interned by a racing call, use that ID instead
dashmap::mapref::entry::Entry::Occupied(entry) => {
// Return the interned ID set by the thread that beat us here.
let id = *entry.get();
// Release the lock on key_map so we don't deadlock.
drop(entry);
// Fixup the broken invariant by removing the vale we just
// optimistically added.
self.value_map.remove(&next_id);
id
}
// We won any races so can complete the interning
dashmap::mapref::entry::Entry::Vacant(entry) => {
let next_id = self.counter.fetch_add(1);
let next_id = Id::from_id(crate::id::Id::from_u32(next_id));
let old_value = self.value_map.insert(next_id, Box::new(data));
assert!(
old_value.is_none(),
"next_id is guaranteed to be unique, bar overflow"
);
// Fixup the broken invariant by adding next_id to the key_map.
entry.insert(next_id);
next_id
}