Fix some missing write barriers and add some helpful comments (#56396)

I was trying some performance optimization which didn't end up working
out, but in the process I found two missing write barriers and added
some helpful comments for future readers, so that part is probably still
useful.

base/runtime_internals.jl

@@ -248,7 +248,7 @@ binding_kind(m::Module, s::Symbol) = binding_kind(lookup_binding_partition(tls_w
     delete_binding(mod::Module, sym::Symbol)
 
 Force the binding `mod.sym` to be undefined again, allowing it be redefined.
-Note that this operation is very expensive, requirinig a full scan of all code in the system,
+Note that this operation is very expensive, requiring a full scan of all code in the system,
 as well as potential recompilation of any methods that (may) have used binding
 information.
 

src/gc-interface.h

@@ -192,6 +192,11 @@ JL_DLLEXPORT void *jl_gc_perm_alloc(size_t sz, int zero, unsigned align,
 // object header must be included in the object size. This object is allocated in an
 // immortal region that is never swept. The second parameter specifies the type of the
 // object being allocated and will be used to set the object header.
+//
+// !!! warning: Because permanently allocated objects are not swept, the GC will not
+//              necessarily mark any objects that would have ordinarily been rooted by
+//              the allocated object. All objects stored in fields of this object
+//              must be either permanently allocated or have other roots.
 struct _jl_value_t *jl_gc_permobj(size_t sz, void *ty) JL_NOTSAFEPOINT;
 
 // ========================================================================= //

src/julia.h

@@ -1138,15 +1138,15 @@ JL_DLLEXPORT jl_weakref_t *jl_gc_new_weakref(jl_value_t *value);
 STATIC_INLINE void jl_gc_wb(const void *parent, const void *ptr) JL_NOTSAFEPOINT
 {
     // parent and ptr isa jl_value_t*
-    if (__unlikely(jl_astaggedvalue(parent)->bits.gc == 3 && // parent is old and not in remset
-                   (jl_astaggedvalue(ptr)->bits.gc & 1) == 0)) // ptr is young
+    if (__unlikely(jl_astaggedvalue(parent)->bits.gc == 3 /* GC_OLD_MARKED */ && // parent is old and not in remset
+                   (jl_astaggedvalue(ptr)->bits.gc & 1 /* GC_MARKED */) == 0)) // ptr is young
         jl_gc_queue_root((jl_value_t*)parent);
 }
 
 STATIC_INLINE void jl_gc_wb_back(const void *ptr) JL_NOTSAFEPOINT // ptr isa jl_value_t*
 {
     // if ptr is old
-    if (__unlikely(jl_astaggedvalue(ptr)->bits.gc == 3)) {
+    if (__unlikely(jl_astaggedvalue(ptr)->bits.gc == 3 /* GC_OLD_MARKED */)) {
         jl_gc_queue_root((jl_value_t*)ptr);
     }
 }

src/module.c

@@ -37,6 +37,7 @@ jl_binding_partition_t *jl_get_binding_partition(jl_binding_t *b, size_t world)
     _Atomic(jl_binding_partition_t *)*insert = &b->partitions;
     jl_binding_partition_t *bpart = jl_atomic_load_relaxed(insert);
     size_t max_world = (size_t)-1;
+    jl_binding_partition_t *new_bpart = NULL;
     while (1) {
         while (bpart && world < bpart->min_world) {
             insert = &bpart->next;
@@ -46,11 +47,11 @@ jl_binding_partition_t *jl_get_binding_partition(jl_binding_t *b, size_t world)
         }
         if (bpart && world <= jl_atomic_load_relaxed(&bpart->max_world))
             return bpart;
-        jl_binding_partition_t *new_bpart = new_binding_partition();
+        if (!new_bpart)
+            new_bpart = new_binding_partition();
         jl_atomic_store_relaxed(&new_bpart->next, bpart);
         jl_gc_wb_fresh(new_bpart, bpart);
-        if (bpart)
-            new_bpart->min_world = jl_atomic_load_relaxed(&bpart->max_world) + 1;
+        new_bpart->min_world = bpart ? jl_atomic_load_relaxed(&bpart->max_world) + 1 : 0;
         jl_atomic_store_relaxed(&new_bpart->max_world, max_world);
         if (jl_atomic_cmpswap(insert, &bpart, new_bpart)) {
             jl_gc_wb(parent, new_bpart);
@@ -548,6 +549,7 @@ static jl_binding_t *jl_resolve_owner(jl_binding_t *b/*optional*/, jl_module_t *
         // changing, for example if this var is assigned to later.
         if (!jl_atomic_cmpswap(&bpart->restriction, &pku, encode_restriction((jl_value_t*)b2, BINDING_KIND_IMPLICIT)))
             goto retry;
+        jl_gc_wb(bpart, b2);
         if (b2->deprecated) {
             b->deprecated = 1; // we will warn about this below, but we might want to warn at the use sites too
             if (m != jl_main_module && m != jl_base_module &&
@@ -740,6 +742,7 @@ static void module_import_(jl_module_t *to, jl_module_t *from, jl_sym_t *asname,
             jl_ptr_kind_union_t new_pku = encode_restriction((jl_value_t*)b, (explici != 0) ? BINDING_KIND_IMPORTED : BINDING_KIND_EXPLICIT);
             if (!jl_atomic_cmpswap(&btopart->restriction, &bto_pku, new_pku))
                 goto retry;
+            jl_gc_wb(btopart, b);
             bto->deprecated |= b->deprecated; // we already warned about this above, but we might want to warn at the use sites too
         }
         else {
@@ -749,6 +752,7 @@ static void module_import_(jl_module_t *to, jl_module_t *from, jl_sym_t *asname,
                     jl_ptr_kind_union_t new_pku = encode_restriction(decode_restriction_value(bto_pku), (explici != 0) ? BINDING_KIND_IMPORTED : BINDING_KIND_EXPLICIT);
                     if (!jl_atomic_cmpswap(&btopart->restriction, &bto_pku, new_pku))
                         goto retry;
+                    // No wb, because the value is unchanged
                 }
             }
             else if (jl_bkind_is_some_import(decode_restriction_kind(bto_pku))) {