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Take a per-object lock in objc_setAssociatedObject, and only to install #409

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Take a per-object lock in objc_setAssociatedObject, and only to install #409

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b5025f4
Take the association lock only where a reference is installed
DTW-Thalion Aug 11, 2026
a4c054e
Take a per-object lock when installing an associated reference
DTW-Thalion Aug 12, 2026
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15 changes: 11 additions & 4 deletions associate.mm
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Original file line number Diff line number Diff line change
Expand Up @@ -54,6 +54,11 @@
* @syncronize().
*/
mutex_t lock;
/**
* Lock guarding the structure of the list. Only the first reference list
* in a chain uses it.
*/
ThinLock structureLock;
/**
* Array of references.
*/
Expand Down Expand Up @@ -135,12 +140,14 @@ static void setReference(struct reference_list *list,
case OBJC_ASSOCIATION_ASSIGN:
break;
}
// While inserting into the list, we need to lock it temporarily.
// While inserting into the list, we need to lock it temporarily. An
// existing reference is updated in place.
struct reference *r = findReference(list, key);
if (NULL == r)
{
auto lock = acquire_locks_for_pointers(list);
// If there's an existing reference, then we can update it, otherwise we
// have to install a new one
std::lock_guard<ThinLock> lock{list->structureLock};
// Another thread may have installed this key since the search above.
r = findReference(list, key);
if (NULL == r)
{
// Search for an unused slot
Expand Down