Depending on luck, g_closure_ref may access closure->vint and observe
different values between reads. This manifests as a test failure in
signals-refcount{2,4}, properties-refcount1, and closure-refcount depending
on timing and re-runs.
Jakub Jelinek analysed this on GCC bug PR119607 after I'd reported it
over there as a possible GCC regression.
The critical part being g_closure_ref -> ATOMIC_INC_ASSIGN -> ATOMIC_CHANGE_FIELD
where closure->vint gets re-read repeatedly, both outside and inside the retry
loop. To fix that:
1. Atomically fetch it the first time;
2. Use the cached read, not a fresh read, of vint in the loop;
3. Use g_atomic_int_compare_and_exchange_full in the loop so we get a freshly
cached vint if it changed in another thread.
Bug: https://gcc.gnu.org/PR119607
Fixes: 834ddd19 ('turned all modifications to the first 32 integer bits in a closure into')
Co-authored-by: Jakub Jelinek <jakub@redhat.com>
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