use atomics in the write barrier slow path (#52463)

Use atomics in the write-barrier slow-path to prevent duplicates in the
remset.

As discussed in #50419, setting
the mark bit is idempotent, but updating page metadata in the mark phase
is not.

Author: d-netto

src/gc.c

@@ -1788,13 +1788,17 @@ JL_DLLEXPORT void jl_gc_queue_root(const jl_value_t *ptr)
 {
     jl_ptls_t ptls = jl_current_task->ptls;
     jl_taggedvalue_t *o = jl_astaggedvalue(ptr);
-    // The modification of the `gc_bits` is not atomic but it
-    // should be safe here since GC is not allowed to run here and we only
-    // write GC_OLD to the GC bits outside GC. This could cause
-    // duplicated objects in the remset but that shouldn't be a problem.
-    o->bits.gc = GC_MARKED;
-    arraylist_push(ptls->heap.remset, (jl_value_t*)ptr);
-    ptls->heap.remset_nptr++; // conservative
+    // The modification of the `gc_bits` needs to be atomic.
+    // We need to ensure that objects are in the remset at
+    // most once, since the mark phase may update page metadata,
+    // which is not idempotent. See comments in https://github.com/JuliaLang/julia/issues/50419
+    uintptr_t header = jl_atomic_load_relaxed((_Atomic(uintptr_t) *)&o->header);
+    header &= ~GC_OLD; // clear the age bit
+    header = jl_atomic_exchange_relaxed((_Atomic(uintptr_t) *)&o->header, header);
+    if (header & GC_OLD) { // write barrier has not been triggered in this object yet
+        arraylist_push(ptls->heap.remset, (jl_value_t*)ptr);
+        ptls->heap.remset_nptr++; // conservative
+    }
 }
 
 void jl_gc_queue_multiroot(const jl_value_t *parent, const void *ptr, jl_datatype_t *dt) JL_NOTSAFEPOINT